A couple of months ago, my X3 got into an accident that required the body shop to replace the wheel and tire. Long story short, they did a terrible job with the tire balancing to a point that when I drive 90+ MPH, I can feel a slight vibration. So, I brought it to BMW, and of course for safety reasons, they aren't going to go that fast to diagnose the issue, and it came back fine. No wheel damage and it passed the test for alignment, but when I drive over 90 MPH I still feel a slight vibration and I don't know how to fix that. It's a minor issue and I don't often drive that fast lol. But when I do because I do have the M50 version, it's a bit annoying to feel it in the steering wheel.

I had experienced the same vibration on my X3M50 as you described, in the exact same way. My X3 had just under 4,000 miles on it at the time and I took it to the dealer and requested to be inspected AS A WARRANTY ISSUE.
They did perform a road test and also checked all 4 wheels OFF the X3 and found one to be off by a whopping 1-pound off weight!

Make sure you go to a shop that has the new and more sophisticated wheel balancing equipment. Take a look at HUNTER ROAD FORCE WALKAWAY wheel balancing equipment, that is more accurate in detecting the imbalance of the wheel/tire. The BMW shop I go to for service has that equipment.

I have driven the X3M50 as before and the vibration is no longer there; the problem has been fixed.