How to program ATmega4809 from Raspbeery GPIO
The ATMega4809 uses UPDI (Unified Programming and Debugging Interface) for programming, not ICSP like the older ATMega328P. You need a UPDI programmer.
There is a project jtag2updi which you can run on an Uno that turns it into a JTAG to UPDI adaptor which is then compatible with avrdude -c jtag2updi ....