Hello,
>Instead of trying to run wsjt-x at the control site, I bought a NUC to put at the radio
Why are you making it so difficult for yourself to run WSJT-X? Just connect your Elecraft K3/0 Mini to your pc at your control site and run WSJT-X.
Unfortunately there is an error in the K3/0 Mini, as input and output (mike and speakers) are reversed both in the USB audio device and the two 3.5 mm Mike and Speakers ports at the left side. This means that the USB audio device can't be used, but the Mike and Speakers ports works for WSJT-X well when connected reversed by wires to your pc. I do my FT8 in this way.
I suggest more owners of K3/0 Mini make a request to Elecraft for having this error corrected at least in the USB audio device.
Regarding COM2 on the rear panel of the RRC-1258 it should be connected to the RS232 port on the K3S (or P3), so the COM2 of the Remoterig box is in use. However COM1 on the front panel of the RRC-1258 is not used by Remoterig and can be used for your KPA1500.
73, Palle, OZ1RH.