Is it doable to create a GitHub action that disables the green merge button unless it has a label of 'Ready For Merge'.
Yes, you can do this. You can use the GitHub Action to query the API for the pull request and fail if it doesn't have the label you want. This will use the checks functionality to make the PR red, which will prevent it from being merged if you apply suitable branch protection rules.
However, this has the downside that you make the PR red until it has that label. There isn't really any way to avoid this, unfortunately.
There may, however, depending on your goal, be a different way to accomplish what you want. For example, if your goal is to allow people to work on PRs that are in progress, you may wish to use the draft pull requests feature. Similarly, if your goal is to run some sort of check (CI, linting, etc.) instead, you could just run that check instead of requiring the label.