fwiw you can avoid the configuration of the .ssh/config and hosts by just defining it in the custom included config as well and using the core.sshCommand variable. for example; i have:

# in my ~/.gitconfig

[includeIf "gitdir:~/dev/w/"]
  path = ~/dev/w/work.gitconfig

and then

# in my "work.gitconfig" file

[core]
  sshCommand = "ssh -i ~/.ssh/my_ed25519"

[user]
  signingkey = ...
  email = my@work.email

Thank you for sharing. I had no idea of the existence of the includeIf directive. Shamefully, I had resorted to just having no global user.name or user.email and setting it on a repo by repo basis. For me, just detecting by directory is plenty, as all of my work related code is within a directory of my companies name.