[Edit to mention web app]
If it would be ok for you to use a server that you don’t need to trust, have a look at EteSync. It’s an open-source, end-to-end encrypted, and tamper-proof solution. The data never leaves you device unencrypted, so the server doesn’t get much information except your ip address and access patterns.
The main author has given two talks about EteSync, FOSDEM 2018 and FOSDEM 2019.
EteSync provides an Android app and a web client, that I believe are stable and used by a lot of people already. The web client uses signed web pages to prove it is the official version. There’s also a DAV adapter for desktop/laptop use, which is still pre-release I think.
You can self host the server (I realize you don’t want to do that) or use the one provided by EteSync. The subscription costs $2/mo (or $4 if you want to support the project a little extra).