Mini Oil Change Interval

With as many mechanical/electrical issues as you're going have with that "boutique" car, I'd be proactive about changing the oil at the earliest possible times. Find some online Mini forums and follow what those guys do religiously.
Do it.

As much as I hate the term, 10k is “the new normal.” European cars typically have over-sized sumps too. Just make sure it stays topped off.

What oil does it take?
Depending on individual conditions for the B48 2.0T I'd suggest between 5K and 7.5K with LL-04 oil. Ideally the oil does meet BMW LL-04 along with MB229.5/MB229.51, VW 511 00 and Porsche A40 or C40. 5W-30, 0W-40 or 5W-40 doesn't matter. The current B48 engines aren't as "brittle" as the former generation N14/N18 PSA engines, but they also have their weaknesses, at least the early ones had (crankshaft/main bearings on 2014/2015 B36/B38/B48), not to mention all DI turbos are hard on timing chains.
The issue when following the long interval on a vehicle in my opinion, is timing chain and related component wear when it gets high miles.
I use to own a 2013 VW GTI I bought new. My wife used it for her business travel car to keep miles off her Audi. She put lots of miles on it.
I sold it to my brother who has 184K on it. The car has had religious 5k oil changes using VW 502 the first 80K and mostly VW 504 onwards.
The engine has never been opened up and still has the original turbo.
run a $10 uoa and find out how it performs at 10k

bmw usa has had pretty conservative service intervals for the past decade