First, welcome to the ACA Forums!
Your mileage range is pretty wide as it ranges from 18 to 55 miles per day (assuming an 18-day ride) so it is a little hard to suggest a route based on that.
I have not ridden the Arkansas High Country Route but am familiar with the Ozarks. While they are not long (maybe 1/4 mile or so), they are fairly steep (10%+) and worse is that go up and down, up and down, all day long. I personally think the Rockies are easier since you can get into a rhythm even if it lasts for over an hour. I do not think there is any Hike-a-Bike on the AHC Route and the gravel roads should be relatively OK except with possibly ruts due to storm run off. I am unsure if there are any rough trails but do not think so looking at the GPX data.
The route is pretty and May would be nice. Since you don't define what hot is to you, I would suggest you check out WeatherSpark.com for great climate data for this route and the Texas Hill Country Route and thousands of other places. Just note that May can have very strong storms pass through Arkansas so be sure to keep an eye on the weather radar.
Since it seems you don't have a problem with gravel, for a route that has mountains, lots of dispersed camping, etc., you might think of something like this:
https://ridewithgps.com/routes/38653597 .
This has lots of mountain scenery, lots of dispersed camping, and you can start in El Paso or Phoenix and at the end of the ride, one of you can take a train, bus, or cheap flight back to the starting point while the other stays with the bikes and then drive back to the ending point and pick up the other rider and the bikes. Note there are a few long (5+ miles) somewhat steep (5+% climbs). You would also have to be experienced in planning your supplies, especially water as this is a fairly remote route. You could add extra miles by going to Gila Cliffs Nat'l Monument north of Silver City, NM. You could shorten the route by starting/ending in Tuscon instead of Phoenix but you would have to take a train or bus back to the starting point.
Another option is to take the Southern Tier Route from El Paso to Del Rio with a side trip from Marfa > Presidio > Big Bend NP > Marathon > Del Rio where you catch the train back to El Paso. Here you could drive from Colorado to El Paso, drop off the bikes at a bike shop or WarmShowers host, drive the car to Del Rio, take the train back to El Paso then the car is waiting for you after you complete the ride so you have not time schedule to keep do to a tickets. This route is not as scenic overall IMO but it is easier (less climbing) and the stretch between Presidio to Marathon is nice. Again, you would need to have planning to keep supplies and water topped off.
Whatever you decide, I hope you have a great trip.