Great updates, Lux-Perpetua!
Regarding the flashing adapter, historically I’ve used a Xnano-416 dev board or a AVR JTAG ICE programmer. But they aren’t super cheap and they do bring some complications (Windows vs Linux, custom avrdude stuff, etc).
So I decided to checkout the pymcuprog project and a super simple and cheap adapter. I think this should be a good universal solution for programming these AVR 1- Series chips. I’ve got the parts ordered. I’ll update when I have more info.