Didn't know about Flarum, yet. As I didn't recognize the forum design, I assumed it was another child of yours. 😅
By OP -- I realize now this was confusing, as the comments don't appear in sub-threads -- I meant Yuchenfamily in this case. I couldn't scroll on mobile to go look up the name while writing, like I did now on desktop.
I guess I'll have to try out the webdav sync and what you can do with it with a windows diarium app on two different machines, then, to see whether that is compatible with what I imagine.
What I'm looking for, essentially, is an etherpad for journalling, something like that.