Stopping by to say this is super cool! I've been pretty obsessed with this idea for a while — here's an analysis I did a few years back: https://github.com/gusgordon/atmospheric_satellite

I got a minimum wingspan of around 8m for self-sustaining flight on the winter solstice with a small payload — pretty close to what you all built.

The biggest problem I ran into when creating this analysis was accurately estimating solar irradiance at different altitudes. Here's the module I built for estimating that, in case you find it useful: https://github.com/gusgordon/airmass

Congrats and good luck!