I do have a video I made a while ago, but I dropped the ball on starting a youtube channel to publish it.

Any recommendations on where I can upload and share a video, preferably one that I could embed in my README.md?

Depends on the size, but I have seen a non-trivial number of GitHub projects that reference an .mp4 served by raw.githubuserassets.com in their readme

I would suspect the cheapest way of doing that is to create an issue, drag the mp4 onto it, and then you have a stable reference to use from within the readme

Just start the youtube.