I’ve heard on stream that DTG are quite sure progress will transfer easily between the new version and Xbox one version but does that include custom liveries or not?
It should be using the same save file. So even if you go back to an Xbox One console, it will have your synced save from Series X.
It did so with both my Madden and The Show when I upgraded. So I don't think there will be a problem at all. It all has to do with how they set up your gamer profile. I guess Sony doesn't keep them all the same across the board.
I recently got a series X and everything was super seamless. I have no custom liveries in TSW2, but I do in Forza Horizon (lots of them) and all came over very easily. Smart delivery is very underrated, it makes everything just work.
I keep hearing the horror stories about, and seeing articles on how to upgrade to and transfer your progress on each individual PS4-PS5 game, and it just sounds so SLOPPY and inconsistent, when I'm used to XBox where at any time, depending which room of my place I am in, I can fire up my Series X, Series S, or my XBox One, and seamlessly continue the save on any game from exactly where I left off on any of the other systems, and it'll run the correct version of the game for each console off the same cloud save.