There are so many opinions some that backed up with repeatable data are truths.
Not disputed: 26 is much better for a RUM less and you can achieve the same with smaller case capacity cartridges.
Not disputed: thicker is better for repeating/follow up shots and if you are going to spend lots of range time at 1,200 yards.
Not disputed: The above two if followed will add weight to the rifle.
Now I'll switch to my opinion mode so you know to ignore the rest.
If it were me, I would not use a RUM for a mountain rifle. I love and have multiple. Absolutely no question that less that 26" barrel is not optimal, creates insane recoil and muzzle blast. It all comes out heavier that most want.
Then there are the optics. For a mountain rifle you want something light and strong. Big scopes for 1,200 yard range time are not what mountain rifles usually wear.
Like everything in life, compromises compromises compromises. Even if you built one specific for a mountain rifle and another for 1,200 yard range shooting there will be compromises.
If I were to build a mountain rifle/1,200 range rifle:
Optic: max 18 power, max 44 objective.
Barrel: Carbon sendero profile, 26"
Action: Your favorite magnum long action
Stock: Carbon, light weight
and
Chambering: 30-Nosler
or
Get a Christensen Arms Ridgeline or Classic or Simmit TI in any chambering you desire. I have shot and loaded for a Christensen. It will do what you want.
Get a good optic, higher end Leupold, Nightforce. There are others but it gets model specific.