Pretty sure that once you're in Steam, there's no way out of Steam.
I bought the game through Steam in early April because they had a sale just like the one right now. I wish I hadn't - but the sale there was too good to pass up. I don't like having layers of other people's Terms of Service and data analytics between me and what I've paid for...
Even just Origin is more than I want in there, but now I have both Origin and Steam between me and my purchase.
That said, I think there is a way to move your game from Origin to Steam - but it's a one way trip or I would move in the other direction. And being on Steam really just means you load Steam which loads Origin which loads Sims... and these other 2 apps stay on while you have the game going happily monitoring your computer use...
.