I've made some minor adjustments. By the way, I will use Linux since it's what I'm most familiar with.

The Sapphire Radeon RX 580 Nitro+ Special Edition is out of stock everywhere, so that one is out. Back to RX580 PowerColor Red Devil Golden Sample.

I also saw they have ASROCK H81 Pro BTC R2.0 in stock. Almost half the price of the ASUS Z270 PRIME A. Stick to the Asus or go with the Asrock?

I will not have more than 6 GPU's per rig since it's good for me, load balancing wise.

Also read that the GPU has 150 watt max. 150 x 6 = 900. Would it be enough with a 1200 watt PSU or is it cutting it a bit close? Or get 2 x 650W-750W (easier to find)?

6 x RX580 PowerColor Red Devil Golden Sample
1 x ASUS Z270 PRIME A
or 1 x ASROCK H81 Pro BTC R2.0
1 x Intel Pentium G4400
1 x 4GB DDR4 Samsung
or 1 x 4GB DDR3 Samsung (if I choose Asrock)
1 x Sandisk 16GB 3.0
1 x EVGA Supernova 1600 P2
or 1 x EVGA Supernova 1200 P2
6 x Risers - v006