The devices I want to keep certain files in sync between are my Linux desktop, Linux laptop, gaming PC, and Android phone. The gaming PC and Android phone are not strictly necessary to keep in sync with the others, but it's moderately convenient to have certain files synced and available on them.

For my Linux desktop and laptop, I will continue to use rsync. I have a justfile that contains commands that sync between them using their tailnet FQDNs.

.justfile

to-laptop:
        rsync -aAXP /home/jas/sync/ jas@laptop.tailnet.ts.net:/home/jas/sync

to-desktop:
        rsync -aAXP /home/jas/sync/ jas@desktop.tailnet.ts.net:/home/jas/sync

I only use my laptop when I leave the house to a location where I have enough time and convenient space to set it up and use for a while. I also use it while still at the house if I'm on my desk treadmill or sitting out on the deck in the backyard. Before leaving the desktop and using the laptop, I would run just to-laptop, and vice versa before leaving the laptop and going back to the desktop.

For my gaming PC and Android phone, there is a Proton Drive client for both Windows and Android, so I have it installed on those devices. I could then use rclone from my Linux desktop or laptop to sync the contents of the /home/jas/sync directory to Proton Drive. I would add the following to my justfile:

.justfile

to-protondrive:
        rclone sync --transfers 16 -P /home/jas/sync/ protondrive:/

from-protondrive:
        rclone sync --transfers 16 -P protondrive:/ /home/jas/sync

I usually tend to know ahead of time whether I will need to use any of the above commands – rarely is there a time when I'm using one device and find myself in need of a file that exists on another device. So this will suffice until Proton Technologies develops a Proton Drive client for Linux.

Regarding Proton Technologies developing a Proton Drive client for Linux: I'm starting to wonder if it's a matter of if rather than when. It doesn't seem to be a priority for them at this time, which I find kind of baffling. It makes sense from an economic standpoint to only support macOS, iOS, Windows, and Android, since the majority of devices in the world run one of those. I'm just surprised that they don't make the Linux desktop a priority out of principle because they otherwise seem dedicated to privacy and FOSS.

I was using MEGA sync for a while, and maybe I'll just go back to using that. They have clients for Linux, and they also seem dedicated to privacy and FOSS. It's just that…I have 500 GB of storage on Proton Drive with my Proton Plus account and it irks me that I can't use it the way I want to.

I'm aware of the celeste app, which is supposed to be a Dropbox-like GUI solution that allows syncing with multiple cloud services, including Proton Drive. I've tried it twice, and it always kind of froze when syncing with Proton Drive. The Celeste app is still under development so there are many rough edges to smooth out, but I believe it uses rclone for Proton Drive anyway.