I searched, but didn’t see this specific topic already posted here. After 3 days of testing and beating my head against the wall, I uncovered a hint in this thread: XIAO ESP32C6 I2C Slave Pins? - #3 by PJ_Glasso - XIAO - Seeed Studio Forum .
“… slave mode isn’t available in the current ESP32-C6 Arduino core.”
No compiler error, it just doesn’t work. Replaced the ESP32-C6 Thing Plus with an ESP32-S3 Thing Plus - works like a champ.
Thought I’d share here in case others run into the same problem. If you have more background or roadmap thoughts, please tag them on to the topic. Thanks!