@grelm If you find out anything more about the ability of the M5StickC to handle 5v IO please post it. Once I have a second unit I will probably disassembly one to see if I can identify any protection on the board. Meanwhile I am looking at level shifters but there is nothing on the market that is easy to interface with the grove connector. May have to resort to making up something inline that uses only 5v to power both the high and low side (with a regulator from the 5V) and see if that works.
New product idea here, you better go and develop it before someone else does!
There isn't any protection, 5v on the io will go directly into the esp32!