This is an interesting article from a couple of days ago about tracking diy balloons long distance: https://spectrum.ieee.org/explore-stratosphere-diy-pico-ball.... Given the tracker can be built for $14 it might be worth it to test a version with just the custom hydrogen enclosure and tracker and see how far it gets.

I also found this blog: https://www.theastroimager.com/picoballoning/pico-ballooning.... His balloon JR14 flew for 507 days and made 6 laps of the earth which is pretty wild.