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.