Unfortunately many four-wheel-drive vehicles are sensitive to tyre wear imbalance, but this is something vehicle manufacturers are somewhat coy about explaining to their customers.

Our understanding of the tolerance is that the difference in tread depth should be no more than 2mm across an axle, and 3mm between the front and rear axles to avoid any issues. Regular tyre rotation front to rear should mean you can get more life out of each set of tyres, as we would expect the front tyres to wear more heavily on your X1. Regardless, it is an additional expense that a two-wheel-drive BMW owner would not have to worry about, but it is worth doing so to avoid the even greater cost of a replacement transfer case.

You can switch to non-runflats which would help the ride quality as well as bring the cost of new tyres down.