I used to ride bmx, but not any more!
I would imagine it would improve your mountain bike skills, even if all you learn is how to do jumps and manuals. Pretty basic skills that I guess alot of mtb riders dont have since they’ve never been into bmxing.
I am 6’2″ so just bought the longest frame I could find, a 21.4″ Fly Pantera. As above, get some 8″ rise bars like Slams, ideal. WetHePeople used to make good complete bikes, dont know if they still do but could be worth a look.