xDrive is confusing to buy things for, you're not alone haha I've been there done that.
the WIDTH of the tire (8.5", 9.5") is essentially irrelevant to the xDrive system, all you need to worry about there is whether they will rub the inside of your fenders when turning.
the DIAMETER of the tire (225) is where it matters. try this link:
https://tiresize.com/comparison/
if you put your front sizes in slot 1, then rears in slot 2, it'll tell you the rolling diameter difference from front to back. this is the number that matters as it must be less than 1%.
it is much easier to run a square setup (same size on all 4 corners), but if you're more of a spirited driver like myself, you may wanna put the extra time & money into getting a staggered setup. I did, & I have no regrets!
Good Luck!!
ps: for reference my setup is 20x9 235/35/20 on the front, & 20x10 275/30/20 on the rear. the rolling diameter difference on this setup is exactly 0%