I'd still 100% recommend using ethernet, with two usb<->ethernet adapters on each end and a cable in-between to direct link the two. It's the most standard easiest to acquire way to link two computers, and it doesn't require any other network to work. Using more bespoke hardware is only going to make this more difficult. Using usb<->serial would also be acceptable I suppose, and require a little less configuration effort, but adapting software to use this is going to be much harder when the existing software is so IP oriented. Whatever the case, having off the shelf connectivity seems like such a win, and I continue to dis-recommend building anything yourself: this is/ought be mostly a software problem.

The one carve out I'd make: if you want to minimize the configuration at the target device, it ought be possible to make a rpi0 device that outputs a usb-gadget keyboard. Continue using one of these IP based software systems to send input to the rpi0, then usb-gadget it out to the target device.

The btkbdd project also might be up your alley: if the target system has bluetooth or can have a (very cheap!) usb<->bluetooth adapter added, making the laptop just send a bluetooth HID to the other device is probably one of the lowest configuration most standard paths available for this work.