The front and rear tires are PREECISELY the same outer diameter.

In fact, the tires on the 18: rims, the 19: rims, the 20" rims (etc) all have the exact same OD.

HENCE, the BMW compact spare that will fit in your car is exactly this outer diameter. It is skinny, but it will work on any "OE BMW Tire+Rim" size combo. So there's that.

I will leave it to you to determine if YOU need winter tires - er tyres- for your locale and use. If you never see temps under 40F, you might not need 'all seasons'. (In looking up your sizes, it looks like you already have summer tires, without issue, so this might be moot.)

Finally, I dont mind run flats on my X5. Its an SUV. I run michelin latitude RFTs. Works for me. I DO NOT care one bit about what BMW says and BMW 'star ratings'. IMO this last thing is a marketing ploy to pay the tyre manufacturers back for their cheap OE prices on supplying new tyres for the BMW production lines. Any blather about 'careful design to work in conjunction with the BMW engineered suspension blah blah blah' is nonsense. IMO

(BTW- rented a bunch of Mitsu Panjero's over the years in Australia- pretty swanky 4x4s for off-road, off-sealed driving. Similar to the shogun/montero)

So do get an alignment check/reset when you fit the new shoes. Here in the US its a $100 job.

GL