I use FolderSync to automate everything about files or backup. Even moving files locally on phone. Most rarely used files and simple backups are synced with Mega, its E2EE for all accounts. For frequently used files I sync them with nextcloud webdav, so I have easy and quick access on my PC too.
Both are ok, but Proton is too weird, also IMAP issue for Linux users makes it even worse. Posteo is probably better service, than both combined.