Everything I've looked at lacks a native client for at least one of my devices or has privacy concerns. Proton would be my first choice if they offered a Linux client.

The other route I need to explore is rclone - it claims to connect to everything. It would need to poll / cronjob to update rather than instant updates like the mainstream options. The downside is uncertainty - if Proton or whatever changes their private API or encryption scheme then things will cease to sync.

Taildrop needs improvement with files over a gig and between other users though.