What you can relatively easily do is extend the 'installer' config with whatever you want to add there, for instance applications and a desktop environment.
It should be relatively straightforward to also mount /home from a separate USB stick partition, but I haven't tried.
Oh yeah, that's not a bad idea and definitely what I'd do if I was bootstrapping a bunch of hosts at once. As it is I set up maybe one new host a year, which means that it's at least as much work to rebuild an installer boot media + my config as it is to do a base install and then pipe a script through bash to finish setup.
Yes, and you can specify your .vimrc, your vim plugins, the users you want setup, their SSH keys, your systemd config, etc.
> Is there some way I can boot-to-git-configuration?
Bootstrapping a system is the one place where you still need to do a bit of work, but it's not bad.
I personally have a script in my Nix config repo which pulls my config down to disk, sets up symlinks, and then switches to the new config. From there everything then goes through standard Nix tooling.