Another 2 cents from me.
I saw you have No.6 in your tsundoku. I finally picked that up I think last year, mostly randomly, and got so into it that I almost accidentally finished it in a day. While it is rated lvl 32, I think that is mostly due to the fantasy/sci-fi setting. I was able to read it very smoothly, and it’s pretty short.
Other possible candidates that I know might be 木漏れ日に泳ぐ魚 (if you’re comfortable reading Onda Riku for long stretches, as it’s the shorter one of her works in your list), 推し、燃ゆ (super short, but very slangy/sub culture centered. Getting into it at the beginning may be harder), わたしの幸せな結婚 (easy slice of life, may be a bit boring to read in one sitting depending on your taste).
So I read two free volumes of 消えた初恋 | L22 this week and really enjoyed it. I was planning on shelving it and reading some of the other books I already own before coming back to it after I’ve read down some of my tbr. There’s also a chance it could go on sale at some point and I could grab it for cheap. But I really want to keep reading it.
Thank you all for the thoughtful advice! Honestly I’m still not sure what I’ll be doing; it’s likely going to depend on my energy levels for the day, though I’m leaning towards manga.
Thank you in particular to @Naphthalene, @bungakushoujo, @nikoru, and @Belerith for the great advice on picking a one-day novel; I will definitely be going back to those suggestions when I’m able to take on the challenge, whether that be Friday or later! (And I’ll definitely have to report back on my thoughts on 秋の牢獄, haha.)