UStik

Gear should be no problem, there are maintenance-free gears which virtually never fail. Maybe you end up with a gear-less solution but you shouldn't exclude a gear in the first place. Maybe you just can't find a low enough kv motor. The mentioned AXI F3A has only 235 kv but is fed by a 10s LiPo battery, which is a good idea. But then it turns just too fast for your project. A gear practically reduces kv, just divide the motor kv by the gear ratio.

The problem of lower air density mentioned by BMatthews is another thing to watch out for but I thought we get the basic points clear first. Aerodynamically, all gets faster with altitude, that is airspeed increases and the prop turns faster. The motor is unaffected by air density but turning faster means giving less torque. So there is just a maximum altitude (ceiling) for the airplane and it needs a stronger drive than the average model to get that ceiling above 15 k ft.

Temperature and moisture effects are yet other (smaller) influences and have been mentioned as well.

I forgot: The "paddles" have some aileron effect but are no full replacement, especially not with a high-aspect-ratio wing. What you need is dihedral for stability and basically a R/E plane. Polyhedral wings handle nicely and effective dihedral should be 9 degrees, that's a good rule of thumb.

To illustrate what I mean: The blue/yellow parkflyer has 3 / additional 12 degrees dihedral on the inner/outer wing panels, giving 8.8 degrees effective. The Motorglider has 3 on the wings and additional 30 degrees on the tiplets, giving 9.0 equivalent. Both are typical shoulder-wing R/E models.

Edit: Naah, I was wrong. I forgot that the average model has a strong drive. Just tried with a calculation for my Sr. Telemaster with a weak drive. It's not exactly overpowered but would still climb at 18k ft (half the ground air density), just with only 58% climb rate and 18% more air speed. It would need about 40 minutes to get at 18k ft, so that's the reason for a stronger drive (which I have).