Handlebar width...
 

Who knew that was a thing?!? Think I'm gonna have to replace the bar on the new MTB already! It just seems much too small; my arms are crossing my chest, feels very restrictive!

Need to sort that!

Advice: Is it worth thinking about a riser?

Sorry. Forgot to do the obligatory "I got a new bike" thread. Thought I'd forego it and start here!

I don't ride a mountain bike as much as I used to (the hills get steeper as you get older), but I've swapped bars a lot on both road and MTBs. Here are a few observations; YMMV, of course:
Back when I used to ride a lot of singletrack and technical terrain, I liked really narrow bars, at one point as narrow as 18 inches. Feels impossibly cramped to me now. I think my current bars are 22 inches (FWIW, I'm 6'4" with a 37-inch sleeve), and they're about right.
On my road bikes, I like wide bars. I bought a 50cm bar cheap off a clearance table a couple of years ago to put on an old frame, and liked it so much I moved it to my Atlantis, which I ride probably 80 percent of the time. The 42-44cm bars I "should" ride feel too narrow now. The moral of the story is that if you're going to switch anyway, think about going outside the recommendations, or at least experimenting.
As for the riser...I'm 67 years old, not as limber as I used to be, and all five of my bikes are set up so my hands are about level with the saddles. I raised them all five years ago, when I was pretty fit, and despite universal warnings, it didn't slow me down at all. I may have gotten faster, because I was more comfortable.

You are forbidden to post until you provide a picture of the correctly posed new bike in front of a white garage door. :p