Another vote for Joplin. The app itself is very good, very intuitive to use, with lots of features. In my case, PC-to-phone syncing took some setting up with Syncthing. Standard Notes is much easier in this regard, but it’s very bare bones if not paid for.
As another alternative to Obsidian or Notion, there’s Anytype. As far as I understand it, it’s not truly FOSS, as their license restricts commercial use. Regardless, I’ve switched to it from Joplin and quite enjoyed it. It does take some time getting used to (the same with Obsidian or Notion), and whether it’s actually worth using productivity-wise heavily depends on your workflow.