Is such a feature even possible for Stardew Valley considering it is mostly being developed by one person only?
CD Projekt Red has cross-saves between Steam, GOG and Nintendo Switch for The Witcher III but it is a AAA game made by an entire team and by CD Projekt Red, the company that made GOG in the first place.

I hope ConcernedApe can add this Cloud cross-save feature to Stardew Valley, and I really hope that maybe the code to it can be given to him from CD Projekt.
I would buy Stardew Valley on GOG, Switch and mobile if such a feature ever happens.

I have sdv on steam, gog, android, switch, ps4 and vita.

That said. Save compatability works from mobile to gog/steam builds and vice versa. For nintendo switch, the save works for exporting to mobile, gog and steam, but is not backwards compatibile due to compression issues (least that I've been able to do) for migrating my save from switch to mobile/pc back to switch.

Dev can do it, whether they actually officially roll out a integrated means to do so, that's a separate issue altogether