3. Cryptomator has native Linux port. They distribute AppImage, but there’s also a Flatpak: https://flathub.org/apps/org.cryptomator.Cryptomator Dropbox itself also has native Linux app: https://flathub.org/pl/apps/com.dropbox.Client

I personally use Nextcloud with self-hosted storage and highly recommend it - again, with native Linux app.

You won’t get official support for OneDrive, Google or iCloud, but there are always some 3rd party clients you can try. I’d advise migrating out of those solutions in favor of something more FOSS friendly if you want to get good reliable experience and privacy.