The beauty of Stardew Valley is that it's going to run on pretty much anything because it just doesn't demand that much from a graphics or processing standpoint. I don't think it's possible to buy a tablet that can't run the game. The one I use is at least 2 years old and cost less than $300. It's called Dragontouch and I've literally never seen any other tablet like it, but it runs my games just fine.
From another standpoint, of course, this is unfortunate news because I didn't narrow down the options at all. Sorry!
So my suggestion is that you go to a physical store and play around with what they've got out on display. That way you can pick one that is the right size and weight for you. Those are important, because I assume you are going to be playing the game a LOT, so you need to be comfortable. My one regret with the Dragontouch is that it is a lot thicker and heavier than I expected, so it is not nearly as portable as I had intended.