I'm actively looking. NextCloud is on my list of maybes, as is S3 or S3-compatible services like OVH. I'm using Fastmail files as a stopgap via WebDav, but it's slow, doesn't give offline files, and I'll need to rig my own backup solution.

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.