Game Input MultipleXer

T300 firmware does not have ffb so I guess you would use G29 firmware.
Since T300 is an official device, there's no config for it. You need to create it yourself.

T300 is the official wheel for playstation 3/4/5. You don't need gimx to make it work, just plug in console's usb port

Error: failed to read packet header from the GIMX adapter

It may be wrong COM port for uart side, recheck which COM port gimx was using.

So in your case:
Gimx adapter, racing wheel and dualshock 4 controller plugged to PC
Hori minipad plugged to Drivehub
AVR side from Gimx adapter plugged to Drivehub
Is that right?

Do re-check if you installed usbdk or not.

Nah, gimx can't control mouse DPI so you need to raise your mouse DPI as high as you can
If you set DPI to 1000 and gimx's config DPI to 3500, gimx still read DPI at 3500 and this value is wrong. So your movement is so slow that's why.

Xbox series S can use wheel from previous generation console like Logitech G920. But G920 firmware has not been finished

I got a ps4 and a T300 wheel. But they are in different room.
I hope I can help you if I have free time

Yes, it must be a faulty UART board.

Yes, I did it and gimx worked again with both of them