I use my own NAS along with syncthing to backup and sync stuff across my phone, laptop, and desktop. Before that I was using mega.nz with its native Linux client, which worked fine sans a weird issue where it’d repeatedly transfer the same file forever.
Way back I also saw a paid 3rd party Linux-native app that supposedly works with all the major personal cloud carriers, though I never ended up using it and have long since forgotten what it’s even called.