Forget about those conventional touring bikes. A better choice for riding mixed surfaces is the Salsa Fargo.
29x2.1 Vittoria Mezcal tyres will roll very nicely on sealed roads or gravel.
You will have the option of using bikepacking gear or conventional racks and touring luggage. They are very popular and stocks sell out quickly, so get you order in early.
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Right. I'll second the versatility of mountain bike frames. The geometry gets the bars up with relatively short steerers (carbon-friendly). The wider stays and forks make for similarly wide selection in rims and tires.
I've gradually replaced my stable with an old Surly Karate Monkey with 3 forks, two stems, and 3 sets of 622mm wheels, 2 sets of which are Rohloff-based - one with narrow rims and one with wide rims. (The other set is a light-weight single speed pair for commuting.)
Forks include the original with steerer cut relatively short, a new braze-on-laden ECR fork with long steerer to accept a Fred Bar with arm rests above my regular bars, and an old Rock Shox Reba air, currently set up at 80 mm travel. Very happy. Packing lightly, I can keep up with just about anyone I meet touring on road or trail.
Have fun with this. And if it's the difference between riding and not riding, don't wait for perfection. My first really big trip was on a criterium racing frame with no eyelets or water bottle bosses, tubular tires, and an 8-pound kit for clothes and camping. What a blast.