I don't see why we shouldn't get access to a Steam key once the game has reached that point in time. We're contributing to you guys to help support future development, to be honest I think it's the least you could do.
We're discussing this with Valve. Naturally, we'd love to give Steam keys to our itch.io customers if Valve allows it, but right now, it seems that even promising a Steam key based on an itch.io purchase when the game isn't available on Steam appears to be a violation of the Steam Terms of Service -- and we don't want to make any promises we can't be 100% sure we can deliver on.
So, we've requested clarification from Valve, and we'll let you know what we find out.
Maybe you can do what Tynan Sylvester did for Rimworld. Once released on steam, Tynan was given a number of keys he could distribite himself to the people who purchased the game way before it hit steam, like me. We could get the keys a few days later, but they were available and Rimworld is still overwhelmingly positive on steam, because nobody could say nothing to him.
I had easily over 300 hours in Rimworld before it hit Steam. I could see dumping plenty in Aven as well, but if you guys manage to get a system to supply steam keys going, that would be amazing, as I would love to hit you up with a positive review. I quite like what this game represents.
Valve has responded and clarified that giving or promising Steam keys for a game that's not currently available for distribution on Steam would be a violation of their terms of service and is frowned upon. Official statement here:
Personally I would prefer a GOG release. That way it's DRM free and from what I understand GOG's cut is smaller than Steam's, meaning you devs get more for your effort.
Whatever you end up doing please don't let one version of the game lag behind another. There is a problem right now of some devs not updating GOG versions in favour of Steam users. Treat us all equally and we'll buy your game where it's best for both us and you :)