> and it was a hassle to get local repositories to use the correct keypair

I agree. The way that I deal with this is as follows:

In my ~/.ssh/config I have content that looks like:

    Host gh-company-a
            User git
            HostName github.com
            IdentityFile ~/.ssh/id_ed25519_company_a

    Host gh-acme-inc
            User git
            HostName github.com
            IdentityFile ~/.ssh/id_ed25519_acme_inc

    Host gh-sponges-corp
            User git
            HostName github.com
            IdentityFile ~/.ssh/id_ed25519_sponges_corp

And then instead of

    git clone git@github.com:companya/foo.git

I'd type

    git clone gh-company-a:companya/foo.git

Likewise, instead of

    git clone git@github.com:acmeinc/baz.git

I do

    git clone gh-acme-com:acmeinc/baz.git

and so on.

With this way of doing it, the correct key pair gets used both for the initial clone and for subsequent pulls and pushes.

I suppose I could make a wrapper program that would take care of the substitution for me, to further reduce the amount of hassle. In fact I might end up doing that. I already have a few wrapper programs for various git commands.