I’m embarrassed to admit that I didn’t try the simplest solution, using the ESP32-C6 config from Espressif ESP32-C6-DevKitC-1 — PlatformIO latest documentation and adding framework = arduino. I’ve successfully uploaded code to the ESP32-C6 with the following platformio.ini:
[env:esp32-c6-devkitm-1]
platform = espressif32
board = esp32-c6-devkitm-1
framework = arduino