I've never understood why they opted for balloons over high altitude solar drones like Zephyr (1) which are much more maneuverable and can stay over a particular area.
1. https://www.airbus.com/en/products-services/defence/uas/uas-...
1. https://www.airbus.com/en/products-services/defence/uas/uas-...
Even today that's challenging - today's 24x7 drones glide all night because that's more efficient than carrying the weight of a battery that can last all night.
https://newatlas.com/aircraft/zephyr-8-mission-ends-abruptly...
Unfortunately, it ended with a crash.
Balloons are a bag that can carry tonnes and last forever in theory.
The obvious choice is balloons.
You can tether balloons for local.
In practice what works best is a hard question.
What will the military fund etc.
Perhaps a happy medium would be a solar airship with a rotor for maneuvering during the day.