Now by all means notion isn't perfect at all, but for me who just wanted to start writing notes it seems the right choice at the time.
I would like to switch to BookStack as it looks great and is a great product, but the one issue that worries me is when it comes to upgrades and migrations. Generally I found when upgrading database based platforms I end up messing up hugely causing myself a huge headache, and then eventually not upgrading at all. For example I used Monica CRM, and totally botched the upgrades eventually just closing it down and using Google contacts instead.
If there was an easy way to solve for that I'd definitely be on board with self-hosting it myself. But at the moment I just don't have the time to resolve upgrade issues :(