In that guide you linked, there is a folder inside the chunithm folder called "手台設定" that has a guide as well as the chusnio dlls and other files for the CFW, just make sure to use the default newio fields instead of the keyboard ones in segatools. You don't need the UART cable, or need to worry about overwriting the base firmware. From when I was installing it, the CFW is for the host and LED FW, which can be done through the USB ports on the tasoller, and you can revert to the original FW for that anytime with no problems. It's only the touch firmware that is installed with the UART cable, which is not needed to install the CFW. The only issue I ran into was needing a different host FW for AMD cpus.