On Mon, Feb 22, 2021 at 12:51 PM, Dave Schwartz wrote:

The CP2102 is also the conduit for the SerialA command channel

In other words, the USB port is used when you want to control OnStep from a PC or a Raspberry Pi.
If you don't use either, then just flash the Blue Pill module with the ST-Link adapter that you are
getting and forget about the USB port.