Kudos to
#Nextcloud for providing desktop file sync on Linux. I know
#Dropbox also offers this, but hosted Nextcloud can be much cheaper and also offers an app suite that includes document and spreadsheet editing, to-do lists and calendar.
AFAIK, Google, Microsoft and Proton do not offer native Linux clients. I'm sure they all make extensive use of Linux servers (though I'll give Microsoft a pass on their backend), and they should make an effort to support Linux desktops.
I'm unsure if there are other services that offer Linux support, and I do know that 3rd-party software to connect to Google Drive does exist but know nothing about it. (Also, I want actual file sync, not WebDAV.)
Yes, I have successfully used
#Syncthing in the somewhat recent past, but file sync to a central server is a different service that is better for my workflow.
The key for my situation was finding a Nextcloud service provider, which in my case is
https://TheGood.Cloud. I'm happy to pay a customer-focused European company to run Nextcloud for me. I can work on my local filesystem, in compatible desktop apps, on the phone and in the browser.
I started this by looking for the best notetaking apps and realizing that it was more important to have total access to the notes and then every other kind of file along with it.