what about sticking to molex cables at least for the risers ??!...
Ther issue with SATA power is the connector. You can get 2xSATA to 6 pin PCIe adapters to solve that issue.
Most are actually 6+2 but I would only use 6.
You can also use the weakest solution on the riser for the GPU that will draw the least power, based on the
card's TDP and the number of power connections.
For example I have an EVGA 1070 FTW that is rated at 150W with 8+8 power connections. Including the riser
that's 425W of supply for 150W of draw. The riser can be expected to draw around 30W and the 8 pin PCIe
around 60W each. Even a SATA riser can handle that.
That's an extreme example but anything below 50W is safe for one SATA connector.