Thank you for the detailed reply, it was very helpfull.
I can confirm a successful flash with v4 firmware!
But, just like you mentioned it, we lost 5G radio. Everything else seems to work perfect, leds included.
So a new v4 build with kmod-mt7615e and kmod-mt7663-firmware-ap is the next step.
Any info or tips is as always appreciated, I'll keep you posted!