Very nice dropbear setup explanation, including key transferral and hardening.

Two remarks:

You can add your key to ~/.ssh/authorized_keys for individual users as usual with openssh - perhaps this is a thing only for newer dropbear versions? The given path would then be for root/system-wide access.

In order to disable password auth for systems that do not have uci, you can add -s as startup parameter, e.g. through editing /etc/default/dropbear field (though probably a better file than in 'default'). see here

Don't forget to restart after operations /etc/init.d/dropbear restart (though it will just invoke systemctl on systemd devices)