@puzzlezaddictyep just done that, seemed to work fine. I should mention that steam seems to see the sims as running for a few seconds, though I might've already mentioned that.
I also then signed out of the EA app and signed in with steam, hoping to kinda force them to be linked, but it didn't work - I did get a different error message! it was 'encountered an unknown error' and told me to restart the app and try again. I did just that, then got the same error message as before.
(I should mention - thanks so much for all of the help, despite it not working you've been very kind)