The TalentCell nearly works, but a miss is as good as a mile. It can power the Pi4, ASI485MC and AZ-GTi simultaneously, but the Pi4 is very slow, perhaps it enters a power-saving state when current is low. Also the Pi4 only boots when the camera is plugged into a USB 2.0 port. Testing indoors with everything connected, with several random slews of the AZ-GTi during the test, the Pi4 captured 250 images before capturing ceased. That's a long way short of the 1200 or more captures I was looking for. I will try out a fast charging power bank, although it will mean toting two power banks around. This is a useful review of various powerbanks, forgive the Nederlands:
https://tweakers.net/reviews/9200/powerbanks-dertien-20ah-accus-getest.html
Gives a rundown of maximum current of 13 different power banks on this page:
https://tweakers.net/reviews/9200/6/powerbanks-dertien-20ah-accus-getest-maximale-stroomsterkte.html