You are using a Vaya (short reach, tall stack) with a steerer tube extender (even more bar height) with a trekking bar (shortest reach possible of all handlebar types). It will be hard to replicate this very short reach/very high handlebar combination on most fat tire bikes. Troll, ECR, KM and DT all have significantly longer reach and/or shorter stack than Vaya, but a Fargo will be pretty close to Vaya. You probably should start your search here, establishing existing reach/stack and then try to find closest fat-tire-capable bicycle match.

Besides geometry issues, bear in mind that a single fat tire likely weighs as much or more than both tires on your Vaya (tube and rim too). Fat bike will feel smoother on rough terrain but the greater rotating weight will likely take it's toll on speed and daily mileage.

You could remove fenders from Vaya, swap tires for 700x50mm (maximum specified tire size without fenders), lower tire pressure and try the bike you already own on some rough routes. It only makes sense to really try with the bike you own before spending $x,xxx for another bike.

Additionally, you might find a Cane Creek Thudbuster seatpost preferable to a sprung Brooks saddle for smoothing out the felt impact from a rough surface.

Lots of folks have ridden the Great Divide route without fat tires - this tricyclist's 3x20" tires probably have less volume and "suspension" than a pair of 700x50s on a Vaya.