Not FOSS but I see so many YouTubers that DM talk about Obsidian for notes. I use it and love it myself, just not for DnD stuff.
Logseq and Joplin are FOSS and are often brought up. Joplins android app is garbage, if that matters to you.
Acreom isn't FOSS yet, but it's on the roadmap and I liked that one.
Notesnook is FOSS but has some features behind a paywall that might be deal breakers for some folks.