Bearing in mind, availability of parts is likely to determine choice. I’d be looking at bars, saddle, seatpost, cranks and cassette. Also switch to tubeless set up.
On of the most overlooked aspects is the mud that will accumulate on your bike during a ride. A crud catcher type device might be a worthwhile investment?
I’ve just done the same exercise with my XC bike, with a similar budget (albeit £1500). I splurged on Ritchey carbon bar, Hope carbon seatpost, Flite Ti saddle and Fox Factory 32 Step Cast forks. I couldn’t find and XTR cassette, but that’s next on the list.