Can you please tell me what to measure with a multimeter exactly and how can I make sure it fits my cabling? atm Im also thinking about what I can do to prevent a clock reset in win10/afterburner/nicehash app
The only thing to measure there is amperage and I don't think multimeter is the right tool for it (yes it can measure amps but you'll have to break the circuit to do so). Clamp meter is way better for that.
It looks like you use a lot of connecting points in your setup, which is no good. Eliminate all of your adaptors. Make/order custom 16awg cables if you need to (but if you use modular PSUs high load might melt/burn connectors on PSU side). Don't power your risers off Sata. Don't run 270W loads via single cable, that's a road to disaster. At the moment one of my rigs rund on a "backup" PSU as I took main ones off for service, I'm short of GPU cables so I run 1 cable per GPU. Even with 240W total per GPU the cable and plugs are significantly warm. Not hot, but quite warm. I wouldn't want to think what happens if suddenly 300-350W will run through them.