The includeIf stuff is pretty neat. I currently keep the SSH complexity in ~/.ssh, where I have several includes, one for each customer|project|identity. Things without unique hostnames, like github, get an alias assigned:

    Host customer-github
     Hostname github.com
     IdentityFile ~/.ssh/customer_rsa
     User git

I have created a tool 12 years ago (still actively maintained) to manage multiple GitHub SSH identities:

https://github.com/dolmen/github-keygen