I'm not well-read on the technology involved. Is it possible (feasible?) to use a high altitude balloon to get a plane to altitude?
You could either detach from the balloon, or use spare solar power to deflate the balloon in case you need it again. Maybe the balloon could help maintain altitude at night.
Perhaps not as high altitude as the stratosphere, but the US was experimenting with deployment and 'landing' of fixed wing aircraft from airships in the 1930s.
https://en.wikipedia.org/wiki/USS_Akron
You could either detach from the balloon, or use spare solar power to deflate the balloon in case you need it again. Maybe the balloon could help maintain altitude at night.