Check your local registration costs. In some states, (Wisconsin), it becomes more costly over #10,000 GVWR whereas in others (California) it is #11,500 GVWR and up where the registration becomes much more costly. I would go F-350 all day if you can. The only differences are a slightly stronger transmission, suspension changes like a larger leaf pack in the rear and a diffferent coil spring up front, and possibly a different rear axle depending on how its spec'd. You're really getting the GVWR rating, which is more of an "on paper" upgrade.
Regarding the axle ratio, I wouldn't lose much sleep over it. I had a 2020 7.3l F-350 with 3.73 gears. It had the #11,400 GVWR option from Ford and had 4,295 lbs of payload. I wanted 4.30s but most dealers do not equip them that way. Unless you plan to run a 37"+ tire, I don't think the 3.55 or 3.73 should be overlooked especially if that's what is available on the lot. Plenty of guys are towing 12-15k travel trailers with the smaller ratios and have no complaints on power - just thirsty as you'd expect. I think this is mostly a result of the 10-speed having quite a few options to find the power band. Regarding the fuel mileage hit, tests have resulted in approximately -1mpg loss from a 3.55 truck to a 4.30 truck. So if you're ordering it, go 4.30 but if you're buying on the lot, 3.55 or 3.73 will likely be fine.
Regarding suspension, my truck had the camper and snow plow prep packages - the stiffest spring rates from Ford both front and rear and you could definitely tell. With a camper in there, it would likely ride much better but unloaded, my truck was rough at times. To combat that in the rear, I planned on doing an Atlas or Deaver spring pack upgrade. there are also cheaper options like Sulastic shackles. In the front, I planned on the Carli coil springs with a 2.5" lift coupled with some shocks from Bilstein or Fox. This would improve the ride and allow for a 37" tire. at that point, I think this platform is a great starting point.
I really liked my 7.3L truck, and only sold it due to the crazy Covid marketplace. I am planning on placing a 2023 or 2024 order for another one. My opinion is it's the best all-around platform out there right now.