I use a similar Dell Optiplex 7000 series.

It boots from the NVME, with an 8TB 3.5 disc for data, and a 500GB SD for my VMs.

I use a splitter on the 12v power line for both of the drives. It’s fine.

This box only has an 80w power supply, and with both those drives hooked up it draws 20w at idle, and peaks at 70w when converting multiple videos simultaneously.