I like your plan alot, I have a Banish 30 that I've been running on a 16" 6.5 saum.
I like it so much that I recently put an 18" barrel on my other Tikka action.
I handload so I necked a 300 wsm down to 7mm and blew the shoulder out to 45 degrees, I'm getting 2800 with 195's.
Since you don't handload I think the 300 wsm with Hornady 200 eldx ammo will be your ticket. It's advertised at 2830 with a 24" barrel so you'll be around 2700 with a 20" tube.
Running the numbers at 2500' that'll get you to 800 yards with the minimum velocity for reliable expansion of 1800 fps.
I wouldn't go over 20" with a suppressor in any situation.
If you're open to build options consider getting a tikka and putting a prefit on.
I've built way too many customs and keep circling back to the tikka action for short mag builds. They're just about the perfect platform for a long range hunting build in my opinion.
Also I personally would go with a ss barrel over carbon fiber, you wont save any weight at 20".
I had mine done with a tikka factory contour flared up to .730 for 5/8×24 threads and it weighs 2lbs at 18"
Here's a few pics of my setup with the Banish in its 9" configuration
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