I have Boox Note Air too, it is being used 70/30 as a note-taking device and tablet. It runs fastmail, k9mail, kindle app, firefox and wallabag just fine.
I wasn't trying to replace built-in note taking app, I think I may have gotten used to it? Honestly at this point I quite like it; it's fast (seriously, there is 0 lag when talking notes), it's capable and it honestly works just fine. I love the UI. I totally understand why you would want to replace it with one note or something like that - I'm just saying that current version works for me :).
I don't trust it yet however - I am using Onyx's sync servers, so my content is backed up without encryption to someone's else servers... Since we are talking about syncing - it also works with syncthing without any special integration; my notes are backed up immediately as pdfs on my other devices.
It is slow, if you think of it as a tablet. There are few options in "app optimizations" menu that allows you to switch between ghosting and page refresh speed.
I'm not sure if you can root it / if the bootloader is unlocked.
I'm using it with different stylus - the one from boox max - it doesn't have that magnetic lock, but it's larger and it feels like stylus tip has lower friction on the display.