Thanks for sharing, Matthew.
That's a very interesting approach, and I'm sure many will find it useful.
I have been using a macro for some time with a similar purpose: Open a file with an alternate app. But my macro takes a very different approach.
It does NOT change the macOS default association for a file, so I can always open any file using its standard default app by double-clicking or pressing RETURN.
But if I want to use an alternate app, I just trigger my macro, where I have setup all of the alternate apps by file ext/type.
If you are interested, you can view/download my macro here:
Open Finder File with Alternate App