Since Quiver is only usable on Apple platforms, I've instead started using Boostnote. It doesn't quite have the same featureset, but it's still a great way of collecting formatted notes and storing them on Dropbox/GDrive.
I came here to say just this. Boostnote isn't 1:1 from what I can tell at a cursory glance of Quiver's features but I've enjoyed its progression even if I barely use it. I've had a rudimentary workflow of just text files in TextWrangler that should honestly get dumped into boost but I'm too lazy to even save the documents most of the time (80-90% of it is temporary ideas) much less properly categorize them.
Yep, boostnotes is pretty good. VSCode w/ some markdown extensions is pretty much the same and is what I use.
I've glossed over their website and can't figure out if it's opensource or not