Yeah that weight difference is no joke. The stock M3P suspension is junk to be honest, mainly the dampers, so if BMW puts some of their better work into the i4 M50 it might handle and ride better from the factory. But I don't think any suspension can fully disguise an extra 1000 lbs, and I think fundamentally the Model 3 is a much better starting point for building a great-handling EV, if you don't mind some modding. (Start with good coilovers, not cheap ones, and that alone will probably be enough for any sane street use.)
Note I'm no Tesla fanboy, in fact I ordered a non-Tesla EV before an agonizing decision to cancel that order and get the M3P instead. I can talk trash all day long about Teslas, I've owned 2 so far and driven many more, but since you seem focused on performance, I might steer you to the M3P. Do test drive it though, make sure you're okay with the noise levels and overall refinement level!
i4 M50 looks like a real nice car, and perfectly fast in its own right. Nothing wrong there. But I agree with everything that in *any* performance metric whatsoever, the M3P is better, or can be made better (handling) than a 5000 lbs i4 ever will be. This includes range. The 2022 i4 M50 range is worse than my 2013 S P85, and Tesla has kept on improving, BMW has some significant catching up to do there.