-Belt and IGH aren't necessarily the best combo to get you out of trouble in harsh conditions, especially of the IGH is an Alfine 11. They may have sorted them out by now, but you have to stay within the manufacturer's recommended settings of about a 2:1 front to rear ratio, because the 11 was notorious for breaking. Often due to guys wanting an IGH, not liking the high gear, going outside of the manufacturer's specs and applying too much torque to the system, before hitting the Internet to complain bitterly. Alfine 8 is more reliable, but think of loaded climbs, because they're both geared really high due to the problem of torque. Get a Rohloff or Pinion if you want to apply torque on an MTB level. The Priority bike is cool and all, I like the belt drive, but it's really a town bike masquerading as a gravel bike with that hub.

-Belt drives had a real moment about 7-8 years ago, but they're a distraction in this thread. Sort of. A lot of the stuff you get on a complete from Surly is of the same level as that Priority's components, just with lower gears because it's a derailleur system instead of a price-point conscious IGH.

-The Surly and the Specialized with a new fork are both great options. Personally I'd go for the Chisel with a new fork, but there are a bunch of good options for a bikepacking bike. Most Surlys, most alloy frame XC bikes like the Specialized with a rigid carbon or steel fork, steel bikes like the Kona Unit & Unit X, Kona Sutra etc. I'd say run what you brung. Don't worry about it all too much and just get some bags for the Chisel and get out there. Work out what works for you, change the fork, see how you like it, grab some cages and dry bags etc. People will poo-poo Aliexpress, but Roswheel and Rhinowalk bags are okay and you can have a decent set of bags for $100 or so. I'd invest in bags, a tarp, sleeping gear and a cook system and just get on out there. You can always upgrade and your bikepacking experience will help you make the right choices.