When trying to find alternative options to notion, I found that a lot of them aren’t that privacy focused but one of the best options I found are capacities.io. According to their website capacities doesn’t claim to be E2EE but say that they don’t share our data and the Usage data (IP, web browser info,etc) are evaluated anonymously. Even though capacities has access to our notes looking past that the rest of the service seems solid enough imo.
Looking for reviews from others since this is a relatively unknown service.
Of course this wouldn’t meet our criteria for notebooks with unencrypted cloud syncing. I don’t know enough about it to address your other questions otherwise, maybe somebody else here has used it.
Have you looked at Anytype? That’s probably the best Notion alternative I’ve seen so far that is privacy/security focused, we’re still considering it: