My only use of GitHub Actions specifies permissions that only allow it to write to the issue tracker, so if the program it executes (which is the "gh" program) is malicious then the only damage it can cause is to the issue tracker and not to the repository itself (unless GitHub Actions itself is compromised). Furthermore, there are no private files (I do not host private files on remote services such as GitHub), so they can't steal my files either.

I think one thing that would help is to support mutual TLS, with X.509 certificate chains for authentication. This is more secure than personal access tokens and allows you to issue them to yourself and others without needing to login first, including to specify whatever permissions you want to set (as a subset of the permissions of the issuer) and to make them expire. (You could also store the private keys of one certificate on a separate computer not connected to the internet, and use that to issue another one to yourself to use that one instead, and can use a passworded private key as well (an alternative of 2FA), so you can recover even if your certificate you are using does become compromised, or something deletes it (intentionally or not), and can revoke the ones that are compromised.)

Mutual TLS (and even ordinary TLS) won't solve everything, but in combination with other things, including ability to restrict access to what is needed, and avoiding needing too many dependencies (since, programs that need too many dependencies is also one thing that can cause many problems), and many other things, it can be an additional step to help with security.