The cross-platform saves probably wouldn’t be that hard from a technical perspective. The game is written using MonoGame and likely doesn’t have that many changes to the core of the game for ports. From what I know about MonoGame (which is admittedly only a little, so I may be wrong!!) the engine abstracts pretty much everything.

I think the stumbling blocks are outside factors. Games often don’t update on different platforms at the very same time because console vendors take longer to vet updates. A game save from an updated PC version synced to a console version without the update could break it. Also setting up and maintaining the server infrastructure for syncing is its own can of worms. Certainly doable but it would pretty much require him to outsource that and he may simply not want that.