Mostly looking for autobiographies/non-fiction in Japanese that are pretty easy to read (N2/N3) that are about:
These are really specific prompts but here are a few suggestions that somewhat come close, even if they don’t fully hit all your criteria:
傲慢と善良 - About relationships in your 30s, also covers loneliness. It is fairly easy (don’t know if it’s N3 accessible but most of the grammar is N2 stuff), but it is fiction and also has a male pov.
ユヒ ナビ・ダーリン - This is a really obscure, older suggestion, but these short stories are from a 在日 Korean writer and focus on the 在日 experience in Japan. ユヒ won the 芥川賞 in 1988 which is neat. Also fiction but semi-autobiographical.
日日是好日 - This time I’m confident I’ve fulfilled the prompt! 日日是好日 is the memoirs of the author and her experience doing 茶道 for 15+ years. She is a professional writer so parts of this are embellished or fictional, but it’s still based on her own experiences and touches on Japanese aesthetics, how 茶道 works & its general philosophy, and the mindset surrounding traditional arts. I absolutely loved this book and it isn’t a very hard read at all (but you can expect to need to look up all the words related to tea equipment and 茶道 that pop up - they are relatively concentrated in the first chapter so keep going if you find the first pages difficult!).
At that language level non-fiction aimed at adults is hard to find. 
However, this is on my TBR and it might fit your “books about not feeling settled” prompt:
I haven’t read it yet, but 負け犬の遠吠え
It seems to not be on Natively yet? But it’s a classic.
Psst @bungakushoujo… a helpful hint! If you simply paste the natively link inline in your sentence and keep typing, it auto turns to https://learnnatively.com/book/07fadc48ee/, which is pretty nice since it has the level. Perhaps you already knew, but as there are a few newer users here, just wanted to mention 
any non-fiction books about japan nature/artisans/aesthetics that you loved
a book I’m reading right now and absolutely love fits this description
猫の浮世絵
it’s a 4 book series about cats in Ukiyo-e (浮世絵) which is a very famous art stream during the Edo period from Japan. Some examples is the iconic huge wave print on a scroll, the kabuki actor piece, Hokusai if you heard the name was an Ukiyo-e artist, etc.
It’s written very “lightly” and in a fun manner, sometimes it just describes fun pieces, and sometimes it goes into more cultural concepts and references (from things like the Imperial Regalia of Japan to local legends regarding a cat that kept Buddhistic scrolls safe from mice during a ship voyage, etc).
She does also mentions some famous schools and goes a bit into famous artists or artists that are attributed to having made an impact yet they’re not as famous.
Of course since it’s drawing in the Edo period you also get a glimpse and explanation into some historical and cultural aspects of the era (which is one of the most famous ones in Japan).
Since everything is around each piece that is shown to you it’s very easy to pinpoint what is what even when you don’t know words that are more technical to the field, and if you read it on bookwalker and have a translate app you can copy-paste things you don’t know to google, or simply look into things that interest you from the book, without much effort (as in you don’t need to start drawing kanji or go through lists from radicals, etc to find words).
If you were open to fiction, Mieko Kawakami’s books fit the first two categories pretty well! I’ve only read Breasts and Eggs, but I know that All the Lovers in The Night would also fit. The only autobiographies I’ve read were by AKB48 idols which I presume wouldn’t really fit what you’re looking for 
totally open to fiction! I’ll check these out thanks.
Would anyone happen to know of nonfiction books covering Perry and his black ships and the opening of Japan to the West?
Shimazaki Toson’s ‘Before the Dawn’ is an excellent book that tracks Japan opening up, starting from around the black ships. However this comes with a massive caveat - my English copy is about 750 pages and weighed about a kilogram (it is tall as well as thick, and the print is not especially big). I wouldn’t personally attempt it in Japanese
Edit: Just twigged that you put nonfiction… Well, there is a lot of historical fact in there so in some ways it is quite like reading a history book! It taught me a lot more than other history books I read on that period.
If you do want to try it, since it was published in the 1920s it is available on Aozora. The print version is four volumes totalling over 1500 pages. (Looks like there are several print editions; probably worth checking them for price and to make sure you don’t get a historical-kana-spellings version by accident…)
I’ve been reading GTO recently and have really been enjoying the story although, I’m not sure if I want to read that many chapters. Does anyone know of a any kind of book similar to GTO?
As a fellow isekai reader (who’s been really enjoying your recommendations in general) I’d love to help you and thought about it long and hard, but sadly everything I checked doesn’t quite fit the bill (or you read it already). 
It also doesn’t help that I’m much more resistant to fan service than you. Most of the time it merely annoys me slightly when I’m not expecting it. I think the following are good, but apologies if I misremember anything.
I might have some recommendations on the manwha side of things (which I read in English). I think all of them are based on novels too, based on comments I read on chapters.
All of the following have “stonks go up”, most have good stories, all are very enjoyable to me, and I consider them among the best manhwa that I read. Only one of them is an actual isekai, but the others have a similar feel. Ignore them if it doesn’t fit the bill enough.
I think you might enjoy 先輩がうざい後輩の話 1 | L20. (Personally I think it’s higher than L20 though; I bought the Japanese version, but realized I had real trouble understanding what I assume is slang, and then read it in English instead.)
Thank you for the recommendations!
I am checking the preview of the light novel of 骸骨騎士様、只今異世界へお出掛け中.
The illustrations at the start had that thing where breasts have a shiny spot (as if oiled up or made of some reflective material like plastic) but the rest of the skin does not. It’s even the case for the maid’s clothes. How does that even work
but if it’s as far as it goes, that’s probably fine.
I’m not really looking for manga at this time, but that looks really cute. I’ll keep it in mind.
I’ve seen that light novel mentioned a few times, including recently. Maybe I should give it a try (although your comments on fan service aren’t exactly promising, haha). I guess I could check the web novel version. I looked up what people said about the differences between versions and the published light novel is basically identical but with extra scenes or details.
I guess I can always switch to the published version if I really like it.
It has a light novel too: 黒猫ニャンゴの冒険 レア属性を引き当てたので、気ままな冒険者を目指します | L29
…which should probably actually go on my wish list, considering how slow the manga releases.
Soooo, I found a comment from 2 years ago that said:
Fingers crossed that’s close enough to the truth. I really like sword dad and Fran, and I hope you’ll be able to enjoy reading about them too!
Oh, nice! I don’t know why it didn’t show up when I looked it up… I might have mistyped it.
Nice!! I was hoping that the extra stuff was the fan service, but I didn’t want to jinx it by saying out loud haha. Thank you!
Edit: I just compared the preview of the published version of 転生したら剣でした and the web novel and there’s indeed some differences in the writing
First few sentences:
Webnovel version:
ここはどこだろう？
目覚めて最初に目に飛び込んできたのは、美しすぎる光景だった。薄暗い空の下、見渡す限りの地平線。その淵から、後光のように光が差している。太陽が昇ろうとしているのだ。立ち昇る来光は、まるで虹の様に煌めき、俺はガラにもなく感動してしまった。
Published version:
目覚めて最初に思ったのは、暗いということだった。
何だ？ 夜？
だが次の瞬間、左側から光が差し込んでくるのを感じた。
俺は光に誘われる様に、そちらに視線を向ける。
そして、俺の目に飛び込んできたのは、あまりにも美しすぎる光景だった。
薄暗い空の下、見渡す限りの地平線。その縁から、後光の様に光が差している。太陽が昇ろうとしているのだ。立ち上る来光は、まるで虹の様に煌めき、俺はガラにもなく感動してしまった。
So, some extra flourish and fixing the Japanese (淵 made me laugh; whoopsie haha)
Since not everyone has read GTO - maybe you’d get some recommendations if you could describe what kind of manga you’re looking for.
Apart from that, some sites that track manga have recommendations for “if you like x, you might like y”, e.g. MyAnimeList. Maybe you can find something there?
I am looking for an easy novel (level wise, early 20-ish? But I think up to early 30s can potentially be ok?) which has an audiobook as well - my plan is to read through the book and afterwards listen to the audiobook.
I am open to anything, although I would prefer to not see too much:
That’s pretty much it!
Here are some examples that fit the bill for me but don’t have an audiobook:
ふしぎ古書店1 福の神はじめました | L23
夜カフェ 1 | L22
I actually wanted to try the reading first and then audiobook approach with ビブリア古書堂の事件手帖 (1) ~栞子さんと奇妙な客人たち~ | L31 but I was too dumb to buy the audiobook from the dwango website And it might have would’ve been too hard anyway, as I’m still very bad at that comprehending spoken japanese 
This might be a good use case to filter by your preferred audiobook provider and search for stuff that way, at least to see if there’s anything you recognize.
Searching by audiobook.jp (they’re DRM-free!) some possibilities that stand out to me are:
Hopefully something there might be interesting? There’s probably a ton more offered through audiobook.jp, but Brandon hasn’t found a way to automate scraping its data, so audiobooks have to be added by hand on natively.
(Ack, half my links aren’t rendering 
)
