Note: I am a happy user, recently paid user, of the Koofr service, but I am in no way affiliated with the organisation, other than just being a customer.
Koofr.eu is a surprisingly nice to use cloud service, an EU based company, with a very attractive, super-userfriendly webUI and apps. Clean and minimalist but support for things like rclone, webdav and even shows documents in Office Online (and allows editing). It has a vault (up to 3) with client-side e2ee.
It would be great if the Koofr API could be integrated/added to the list of Online Accounts. WebDAV is a bit of a compromise.. Koofr does have an app that can be installed via their bash script in userspace, and runs in the background, with only a webservice available to show a UI in a browser.
The Koofr API is already integrated by rclone.
It seems other services also leverage the Koofr sync API like DigiStorage in Romania.
Documentation can be found here: Koofr - Developers