Recently a guy (kerogit) ported NuttX to AVRDB, then I found a low cost board to test:
https://www.electrodragon.com/product/avr128db64-mini-develpment-board-avr128/
This chip use UPDI interface to be programmed, fortunately there is some option to use an Arduino Nano as programmer, as explained here:
https://w2.electrodragon.com/SDK-dat/avr-sdk-dat/UPDI-dat/UPDI-dat.md