What I did for programming the ATTiny1614 is use a generic USB<->UART cable (same that I used for programming Arduino Pro Mini boards) with a 4k7 R connecting RX and TX together as the UPDI pin and passing 5V / GND through as-is.
I also used this command to write the binary:
pymcuprog write -f diyBMSCurrentMonitor_ATtiny1614.hex -d attiny1614 -t uart -u /dev/ttyUSB0 --verify