Questions coming from a noob

Hello guys, kinda new to the whole mining thing, how i regret that i stopped mining back in 2009 when i first heard about it.

Anyhow i will come to my problems.

You all know that finding the hardware isnt that easy, and especially in my country where you get hoards of people from other countries going to buy gpu's and mb's.

Anyhow to make long story short :

I bought 8x rx580 ( yes i was ambitious enough to make a rig with 8 gpus, and i thought i could pull it off )
a msi z270 krait, that had 6x pci and 2x m.2
A celeron G3930 , 8gb ddr4 and an ssd, the rig would be powered by 2 rm850 corsair psu's.

Well when everything came, apart from the m.2 to pcie adapters, i made a wooden frame to fit everything together, and i would later install the other 2 gpus when the m.2 adapters came.

So connected everything, the machine booted up with no problems, updated bios, installed latest amd drivers yada yada, only to find out, that even though all 6 cards were shown, only 3 were working.

Started messing with things, such as changing the peg to gen1 and enabling 4g decoding, only to make it worse. In the end, the board wouldnt boot over 3 cards, and that pissed me off so much i took it back.

So i thought, maybe a little more expensive board would make the deal, i went and gave back my z270 krait, and i got a gigabyte aorus z270x gaming 7 board.

This damn board has no 4g decoding option, and has no pcie configuration whatsoever, and i still fail to boot over 4 gpus, and i did that after like 50 reboots.

Now i want to understand what went wrong, and i am starting to troubleshoot stuff, but without finding answers, as everything seems random.

First of all my gpu's are msi rx580 gaming 8gb, and i flashed them with a custom bios that changes some of the timings, and overclocks the memory from 2000 to 2150 ( that effectively changed the cards from 22mh/s to 30mh/s and with a little undervoltage of the gpu i can get 100w per card, with 30mh/s )

Might it be that the cards are flashed and the pc wont boot ?

Second of all, my psu's are connected to each other with a cable, that when i click the button, it boots both up, psu1 has 4 cards and mobo/cpu/4 fans, and psu2 has 4 cards and 4 fans.
What is retarded about my psu's, is the fact that they came with 2 pcie power cables, that only had 2x connectors on them, so in 1 cable, i use 2 cards, but if they had 4 cables, i would ofc use 1 for each gpu.

Might it be that the cards arent getting enough power and the pc wont boot ?

The risers and all the equipment is brand new, i am an experienced pc builder, but this is driving me nuts.

My other option, is to get this new board back, get 2 less expensive ones, and run 4-6 cards on each rig, but i dont really want to have 2 different rigs running.

Unless what i want to do isnt plausible, i might take it back and go for the 2 rig option.

PS : If i undervolt my cards to what i saw is stable 100-110w per card, can i run a maximum of 6 cards on that 850w psu or am i pushing it too much ?

Thanks in advance on anyone taking the time to read my essay, i would gladly pay someone to help me up, if this is what it takes.