Storing Code Signing Certificate
From what I can see you have done everything correctly, you have stored both the certificate material (i.e. the PFX) and the password for the PFX in a secure location that presumably no one else can access.  Assuming you don't give anyone else permissions on your Azure tenant and you don't blindly accept pull requests that change azure-pipelines.yml then you are in a good place.

If you are feeling paranoid, then you could create a second pipeline using a private repository that is triggered when the previous pipeline completes successfully.  However, IMHO, I would carefully consider the risk you are exposing yourself to against the complexity and fragility of trying to tie two pipelines together.