Very limited choice, yes.
I’ve always found Camskill good for tyres but even they come up short with this size.
I use my original 18” wheels for winters, and the choice for 18” 275 rears was also very poor, both for winters and cross climates.
So much so, that I bought another two front wheels and use a square setup for winter.
As an aside, some will say that BMW only recommends square setups for winter use, and could account for the lack of choice -particularly OE.
The tyre pressure sticker on the b pillar also suggests that.
I’ve not found that to always be the case, although it does make sense.
The choices of tyres are much better with same size all round, and you can still fit front sized tyres to wider rear wheels, although they don’t look great, so you may want to try the same with your 19” wheels or buy another two front wheels, although not ideal if you want to use this all year round.
I may look at cross climates next time (still only for winter use) as our winter weather temperatures nowadays are too up and down for proper winter tyres.