Obsidian. Very simple, all functionality is free, offline, all notes are accessible on your PC, you don’t even need Obsidian to open them because they are in Markdown. Has tons of community plugins for every need, even a Notion-like tables plugin. There is an android app which looks fine but I don’t use it much so I can’t say anything about it. But since all your notes are offline you have to think of ways to sync them. There is a paid Obsidian sync which you dont have to use or you can sync with any other method (manually, github, syncthing, etc…)

Edit: Oops, just realized the post is in FOSS community and Obsidian doesn’t seem to be open source. Sorry! I’m not going to delete the comment though, maybe it’ll help someone