Obsidian is my fav. It’s not FOSS but it uses a flat folder of plain markdown files so it’s very portable and open.
Notion is decent as well but smells like at some point it will enshittify. If you are okay with the pain of jumping platforms eventually you could give it a shot.
Joplin is FOSS and I’ve heard good things about it but idk where it stacks up against the other two.