If you're looking to do this without OS X, or you want to do something like this in an existing Video conference app like Zoom or Jitsi (I have no idea if Mmhmm does or not, they give few details and I can't download it), this is pretty easy to do in OBS Studio. You can do video capture of your presentation, yourself, have background images, and arrange things however you'd like. Here's a Youtube tutorial [1] with some examples. You'd then need a virtual webcam plugin to use your OBS Studio stream in other software. On Windows you can use OBS Virtual Cam[2], on Linux you can use obs-v4l2sink[3]. On OS X I think there's still an open bounty for a OBS Studio webcam solution, so since Mmmhmm is only available on OS X anyway that might be a better solution for you.
It certainly seems like Mmmhmm makes this easier, and allows you to smoothly transition from one format to another, but OBS Studio is very workable as well, and works on more platforms.
1. https://www.youtube.com/watch?v=hk7W9ECtpZE
2. https://obsproject.com/forum/resources/obs-virtualcam.949/
3. https://github.com/CatxFish/obs-v4l2sink