I don't think one could just "throw" something 70k feet up in the air.

Climbing to altitude is the straightforward part. The transition from zero knots to the stall speed of the aircraft (minimum speed at which it can remain airborne) is the tricky bit. Designing for a lower stall speed necessitates wings which produce higher drag (by being larger) which requires more propulsion, which means bigger batteries and motors. So launching from a catapult or rocket or mothership or whatever means a lighter plane.

Launch on a hydrogen balloon

Launching by rocket means the plane would have to be pretty rugged to survive it. And that means more weight. You also have the issue of deployment. Folding wings means more weight and more things to go wrong.

Spinlaunch would disgree but here the trouble is getting off the ground; Take-off is the most demanding phase of flight in terms of propulsion demand.

Spinlaunch would crush any aircraft that is light enough to stay aloft indefinitely.

Lee waves? Thermals? Cumulonimbus thermals? :)

Or balloon launched, parachute landing.

Other options that might work: * Launch from the roof of a vehicle * With a glider winch * Towed by another aircraft * Auxiliary engine / batteries that can be jettisoned & parachuted down