~/.gitconfig:

    [credential]
        credentialStore = secretservice
        helper = /usr/local/bin/git-credential-libsecret
    [credential "https://github.com"]
          username = akdor1154
    [includeif "gitdir:~/src/mywork/"]
          path=~/src/mywork/.gitconfig

~/src/mywork/.gitconfig:

    [user]
        email = "12345+akdor1154-mywork@users.noreply.github.com"
        signingkey = ~/.ssh/id_rsa.mywork.private
    [credential "https://github.com"]
        username = akdor1154-mywork

The `username` is passed to the global secret helper and is thus used for it as part of the pick/prompt context for password. I'm using the libsecret helper but this works with any.

You could probably write a custom git credential helper that does that, if it doesn't exist already.

I use the technique in article for something similar that solves the same problem. I'll find it on pc in a sec.

Do you need multiple files? Since you can store credentials for multipl users and host combinations in the same file.