I just tried it again, and unfortunately there still seems to be no driver support for the ch340g. I'm using Mac OS 11.3 (20E232) on an M1 Air 2020. I see there's now an upgrade available to 11.4, I will update and try again to see if support has been added.

Unfortunately, I have not. I switched to using an FTDI-based USB Serial adapter instead. Are you experiencing the same problem with CH340 devices, where they do not appear (or appear only occasionally) on M1 w/ Big Sur?