TinyCore32 works like a charm. Have ported the BSP from the Author's as well as from Dr.Azzy's. Currently, using with Serial & 4.7K config programmer. Will further work on ProMicro hack for Atmel mEDBG. Great potential for such product.
Using this UPDI programmer is quite simple. I was very excited to program the ATtiny1616 from Arduino. I had no success with other adaptions of a UPDI programmer. Built a interface board to connect cables to the UPDI connector on my boards, and the Tiny Core programmer does excellent with my boards. If you are having problems with other UPDI programming devices, I suggest that you acquire one of these boards. I purchased the programmer with one each of Tiny Core dev boards where all the pins are brought out to pads. The use of wire-wrap wire allows me to place and solder wire into the smaller holes in the board. This is a very well designed board with a USB-C connector to outlast other USB connectors, and an adaptor for esp-01 boards. I wash that I had purchased the boards earlier and saved myself heartache trying to use Arduino with UPDI. I have had questions about the board from TinyCore, and they were quick and cordial in responding to my questions.
I bought two units of this board. They fit perfectly and the Attiny 1616 is a perfect choice, as it supports I2C. The documentation is great too. Would love to see an even longer list for programmers for this board.