Just to confirm: it sounds like the batteries / the charger module v1 + battery combo is not strong enough to power through the boot, but once the system is booted and reform-power-daemon is running, you can unplug the power supply and it’s stable, or no?
Edit: do you already have a ticket open requesting charger module v2? It should be interesting to send you one and to see if it alleviates the boot issue, but it could also be that the cells are too weakened, and in this case (other than replacing with fresh cells) we need to come up with a way to limit the CPU+GPU freq early in boot (i.e. change the default governors, that should hopefully not be too hard in kernel configuration).