I've done some in-car testing with a USB meter:
The A129 draws <0.2A during boot, then 0.5-0.7A whilst recording
The Mobius 1 draws 0.3-0.45A
The Mobius Maxi (capacitor) draws 0.2-0.5A
The Mobius Maxi (battery) draws 0.25-0.5A
So my peak current draw across all four cameras plugged into the 5-way USB adapter is 2.15A. That's about a fifth of the stated 10A maximum output.
I can switch to using the Viofo adapter, but my cabling is much tidier when they're all in the centre console and it's easier to swap them into the powerbank for ad-hoc parking mode.
All three Mobius cameras have started recording long before the A129 is done booting up.
I have reset the A129 to default (MOD 6Uk) and reformatted the card. If that doesn't fix it, I'll roll back to 7Uj that I was using before I noticed these startup problems.