I decided to abandon the ESP32-P4&-C6 board regarding Thread. They work fine with Matter-over-Wifi and that's where I'm leaving until I get a greater sense of success. The -C6 comes from the factory with WiFi, not with Thread. Flashing the -C6 is a one-time shot. If the .bin isn't perfect, it's not exactly bricked, but it is a soldering effort to recover. Flashing the -C6 is via the -P4 and the SDIO that is configured between them. The image sent to the -C6 has to retain that in order for the -C6 to be visible to the -P4. My confidence in the examples offered isn't high.
So, I decided to change direction and got an ESP32-C6fh8 (8mb flash vs the more standard 4mb) and I got THAT to pair to my Hubitat C8-Pro T via Thread as a Dimmer (again.)
The 8b of flash has 70+% free, so it can work on an ESP32-C6fh4. I don't know if that leaves enough for a more interesting Project, but I assume so. On the other hand, there's a reason the h8 exists 