The RGB output on the board is organized through FAN connectors. But I only need one signal pin, I will take power from the additional power supply.Do you take just the power from there, or do you use signal pin as well? Either way, you can take a look at additional LED controllers. For example Klipper Expander board has RGB port and installs as another MCU. Or look at WLED controller, Moonraker has support for WLED.
Or do I not understand something?
It is easier then to transfer the head to a CAN connection, thus freeing the control pin for the RGBW on the board