I'm a senior engineer, made my own renderer and software "suite" to go with it as a hobby project for the resume, but I'm not sure what else I can make to get my foot in the door. Do you have any general advice?
In my opinion, don't write your own renderer unless you can really invest a lot of time into it. There are countless "spheres in a Cornell box" path tracers out there, you'll not excite anyone with the basics. Contribute to existing software like Blender or Appleseed instead, that will give you a lot more visibility and demonstrate that you can write robust code.
Companies like that are constantly staffing up and laying people off with the film cycles so it shouldn’t be too hard to get in.
Once in you just have to keep your eyes peeled for a good job in an abusive industry.