I came here to see if anyone else suggested Simon Tathams puzzles - it’s a real gem! Perfectly dull, all-quality puzzles with no bullshit!

Seconding Obsidian - it’s not FOSS, but the files are just markdown, nothing special, so you’re not locked in. Self hosting is real easy, you just have to Sync the files, and everything follows. I use syncthing between my laptop and phone and am having a good time with it.