After talking to some more people I found out that my endpoint URL was wrong and it was missing the /repos/ part of the URL.
I also ditched the fancy yaml for generating the call for the deployment key and opted for just using a shell: and curl inside it instead; it wasn’t too bad, but I needed to make sure that the variable that stored the content of the key had .stdout appended to the end of it since otherwise it would print the contents of the dict.