Please don’t suggest fiber runs, as the cable is already run and I don’t plan on redoing it.

You said you have conduit, right? Running fiber would literally be as easy as: unplug Cat6 -> tape fiber to end of Cat6 -> pull on other end of Cat6 -> done. If that takes you 5 minutes you’re pulling too slow. Then, just throw on a couple of SFP adapters and you’re solid. No more worries about ground loops, greatly reduced chance of a lightning strike frying equipment on both ends. I think that’s worth a few minutes spent pulling fiber.

Mail server or anything using RDP.

I’ve got 9 of the 8TB IronWolfs in my server, and they’ve been great. The 3 oldest are 4 years old now, and I’ve grown the size of my array as I could over time, and switched from RAID 5 to RAID 6 for the dual redundancy. Speed is not important in my environment, so I can’t speak to that vs WD or Hitachi, but they’ve been fine for me.