Hi Felix,
Sorry for the delay... my STAMP_PICO project was momentarily shelved but I just resurrected it and found your response.
I will definitely try your suggestion since I would like to keep the native port to flash but I need another UART.
Thank you!

Is there a way to access a second hardware UART on the M5Stamp-PICO?
I do not need the SPI port but I do need the UART (and I2C) and uploading code while also using the UART in the application is really inconvenient.
Any help appreciated.

I had weird problems like this with an M5Stamp-PICO until I put more capacitance across the bus (5V and 3.3V). I used a couple of tantalum capacitors I had on hand, not sure the value, probably 33uF. I was powering the device through the USB serial adapter and apparently it was power limited.
I have had no problem whatsoever since then.

Thank you, that is good news.
Now I just have to find out how to configure and access it from the Arduino IDE :)

Does the M5Stamp C3 have an I2C interface? I do not see any reference to it on the pinout.