Yes, ATX_12V_2 is necessary.
Yes, ATX_12V_2 is necessary.
Check if it's required to use the adapter. Your PSU lacks 4-pin connectors.
You likely don't have the right CPU. You mentioned a Corsair RM750x with multiple EPS12V cables, but the 8-pin connector splits into two 4-pin ones if you're only using it as a 4-pin connector.
Standard board and CPU choice probably isn't critical, just an 8-pin connector should suffice. Power rating should be around 235W continuous, unofficial sources suggest up to ~340W, but avoid continuous operation. Look for a CPU capable of reliably delivering about 250W+ during steady use.
You don't need to be divine to understand the specifications of a PSU.
They divided into two connectors. My PC felt a bit quiet. I believe that was the cause.