I cleaned out the local Apple store a few days ago, and now have four different systems - Intel / Mojave, Intel / Catalina, Intel / Big Sur, and M1 / Big Sur.
I’ve been working through the serial transfer code for the past two days solid, and I now have what appears to be reliable communication on the first three systems (I haven’t tested M1 / Big Sur yet). I’m going to get builds posted online after doing a bit more testing.
I will post a link to those builds to this thread so anyone who wants to try them can do so. If you do, please give feedback as to whether it worked for you or not, and with what kind of system, so I can figure out if we need more coverage or not before releasing into the wild.