I recognise those photos Bungo.
Yes, only one other cab can be added to the combo. Or 2 cabs when the head is detached.
Always use 8ohm cabs and your safe.
Sounds like your doing all the right things to reduce feedback risk.
Outdoor gigs usually mean a hollow stage, that can be a problem too, with vibration causing feedback issues.
Do it.. jam it behind the tailpiece with a bit of foam, that also helps stop it vibrating as a potential cause of feedback. Used a G75 for years, now upgraded to a Shure.
Buy the Platinum Pro. It does exactly what you need. The Jtone pickup is a cheap pickup, there are many options out there which will improve your sound.
Sounds like the strings you are using are too low tension. They may be easy to play but the trade off is that they will be quieter, less defined and buzz occasionaly.
Totally recomend the Line 6 stuff.
The G30 never failed and G70, now discontinued, still has more features than any other unit out there.
Any digital wireless works fine with
Piezo or mag pickups.
Its a great bit of kit but screams out for a couple of improvements.
The tuner should be accessed by a long press on the footswitch with the A/B function a quick press.
Also a USB charging port should have been put in the power lead.
HX works very well with piezo on upright. Especially when you load some Impulse Responses from 3Sigma Audio.
In the end there are too many oportunities to tweak parameters on the HX, drove me mad.