Tried doing this with Proton Drive one time. It broke in several different ways simultaneously. Doesn't seem to be ready yet, but maybe some day it will be good for this.

Wait, you can mount your Proton Drive with rclone? Hah, never knew. It's in Beta apparently though: https://rclone.org/protondrive/

it was very slow, and I had to re-authenticate often. I want something like dropbox daemon, which mounts drive and syncs it. Set up once and forget. Since I have not found anything like this for Proton I am still using Dropbox for some synced data.

Isn't rclone copying everything rsync style (at least for Proton) ?