Instead of keeping a personal diary, what about keeping a reading log? You can summarize what you’ve read, write a review, or write personal thoughts and musings about it, depending on what you feel like. And it could potentially also help you retain new vocabulary from reading.
For corrections, have you tried lang-8? I used to use it and remember it feeling fairly reliable. Especially if you correct other people’s writing in your native language, then your own writings get sort of bumped up in priority. I haven’t used it for a while but I remember really liking it!
Eta: didn’t realize this was originally posted months ago! Anyway, maybe you are still open to suggestions. Either way I hope you’ve been able to find something that works for you!
A reading log is something I never thought about. It might be difficult for me to write much in Japanese about it atm though but I’ll definitely keep that in mind.
This is still valid, my writing, other than a little bit of introduction info in Japanese, is mostly limited to textbook answers being written out so I’m happy for any suggestions. They will always be handy later if I can’t use them now.
I haven’t tried lang8 but have tried Hellotalk and also Journaly site, the concept is great but I think my writing must be boring or get punted further down the list as it rarely gets marked or corrected.
I’ve used https://langcorrect.com/ for a while (only stopped because I got too lazy ) and usually my (really boring!) Japanese texts got corrected by Natives in like a day at most,. So maybe try that
As an aside, the translation feature on HelloTalk seems really good, from the bit I’ve used it. Unfortunately its usage is limited, unless you have a paid account
) and usually my (really boring!) Japanese texts got corrected by Natives in like a day at most,. So maybe try that 
