Circuitry choices
Since the new C3 variant carries most necessary protections onboard, the main PCB can be left fairly basic in terms of protection.
Cable considerations
A one-to-one JST-SH 4 pin cable is used to connect the daughterboard to the mainboard.
Make sure that the cable is properly one-to-one; a cable which inverts the pinout will result in reverse voltage applied to the main PCB resulting in its destruction.
One good option for testing and low quantity boards is the Sparkfun Qwiic cables; these are confirmed functional and compatible with the Unified USB Daughterboard series.
Digikey carries both the "correct" and "incorrect" variants of the JST cables in various lengths; before ordering, check the specification/drawing PDF for the item and verify that the cable is of the correct type.
The correct type is the 4-wire variant of the "Socket to Socket, Reversed" type cables listed here.
Make sure that no components block the zone in front of the JST connector; this will risk the cable not fitting or the part(s) being destroyed when attaching the cable.
Connector part and pinout
Use a 4 pin JST SH/SR series connector. The part number is SM04B-SRSS-TB(LF)(SN).
The part can be found from many sources, such as LCSC, Digikey, Mouser, and similar.
Example
A footprint can be found in my random keyboard parts library.
The pinout is as follows, viewed from the top of the connector.
This pinout is identical on both the daughterboard side and mainboard side.
Electrical considerations
The JST SH/SR series are not rated for more than 1A@50V. There is a problem if 50V is coming out of the daughterboards anyways, but do not attempt to do ridiculous, dangerous things such as pulling 10A of current for RGB LEDs over the JST connection.