Just wanted to say thank you for the instructions, they worked really well. I can add that this is how I added the armbian.gpg to my install:
sudo wget https://apt.armbian.com/armbian.key -O key
sudo gpg --dearmor < key | sudo tee /usr/share/keyrings/armbian.gpg > /dev/null
sudo chmod go+r /usr/share/keyrings/armbian.gpg