It has a Ryzen 9900X, 64GB of DDR5, AMD Radeon RX6600XT, 2x2TB Hanye NVMe, ROG B650 ATX motherboard and 850W power supply.

I bought the system mostly to increase the single core performance from the Ryzen 5 3600 I had before. As well as to get rid of all the little 256GB SSD disks I had in the previous one.

850W is an overkill and will affect efficiency. I'd go with less power.

Then you suddenly want a Nvidia GPU on the side and you need more power.

Is 850W really "overkill"? I don't know. I suppose it depends on how likely it is you will expand and add devices.

If I go look up an arbitrary 850 watt supply (seasonic focus gold), it looks like it wastes about 10% plus 5 watts anywhere from 0-60% load. So extra capacity doesn't hurt that one.