The others are generally not supported on Linux. Dropbox has a cli client here that mostly works. At least if we are talking Google/Microsoft. Or the companies are small enough to go under overnight and leave me in a lurch

If long-term is your game and you don't like the big boys, you should probably look into Syncthing. Being an open protocol, it will never go away.

Google drive unofficial client works pretty well for me on Linux.

There is a difference between official and unofficial support. Also, Dropbox is big enough to be around for a while, but this is their product and if it goes south, so do they. I don't trust Google to maintain any product before rebranding, pushing the work to the users, or dropping the product. They have shown otherwise.

Insync works well for Google Drive on Linux.

It did. Recent versions add the suffix xml to files with xml contents, unfortunately.