New CT4S owner here and also annoyed by this issue. It makes the first few moments of every drive a chore, rather than a pleasure.
I tried this workaround, and it successfully stops carplay starting automatically, but I can't seem to get carplay working manually by joining either of the car's wifi networks. Trying to tap the carplay icon next to my device in PCM doesn't help.As a work around, I ended up turning off the "auto join" setting for the Taycan wifi network on my iPhone. At least this keeps the PCM from switching to Car Play every time I get in the car. The downside, is I have to manually activate the wifi on my phone when I really do want to use Apple Car Play.