Thanks to the 200MB per game cap on cloud saves x4 basically can't sync if you have more than even one save. Further, even if you turn off cloud saves for some reason galaxy seems to still attempt to sync. The makes starting the game take much longer than it should.

Is this ever going to be fixed? On either side I mean, GOG or Egosoft.

GOG is the only side which can fixed that.

Egosoft talked to GOG when that limit was announced, but it seems GOG did not provide a solution.

Just use a USB SSD to copy your saves to as a backup, less hassle / less bandwidth use.
I dont know what can be done about the GOG Galaxy issue though, are you sure you turned off Cloud saves in the settings ?, if you are it may be worth submitting a report ticket as its clearly bugged if thats the case.

Gog is unfortunately becoming more and more of a problem for my games, Cloudsave x4 doesn't work.
Craft the world new Dlc does not appear, why nobody knows.
Communication about such things and why there are less and less releases on GoG, none.
I hope and wish that Gog recovers and addresses this, otherwise ... I think they will go under and that would be fatal.

There are two places I found to turn off cloud saves - globally and per game. Neither seem to stop the "syncing" on launch in my experience.

Also saving to a personal cloud like DropBox, OneDrive, GoogleDrive, etc can work fine if one has a good bandwith.

But I have used a USB too with games with no cloud saves, as you suggest.

That said, it is a pity that X: Rebirth has no cloud saves :(