Use SSH for access to private submodules?

[gleicher]

Can I use an SSH key to access a private submodule?
Is it possible to use different SSH keys for each submodule?

I have a project that works fine using a personal access token, but I would prefer to use an SSH key

My setup works fine with the personal access token:
token: ${{ secrets.PERSONAL_TOKEN }}

But this doesn't work:
ssh-key: ${{ secrets.DEPLOY_READ_KEY }}

I think the problem is that each repository (and there are multiple repositories that are are subrepos) needs to have a unique key (If I read https://docs.github.com/en/developers/overview/managing-deploy-keys#deploy-keys correctly).

Is there a way to do this?