Yep. I did everything just as you wrote. Thanks, by the way, for all your help!
I got as far as running MakeMKV, but nothing I could do would allow it to find my BD drive. And that's strange, because it's installed in the workstation, and other programs (like the Leawo video converter on Windows) can find it.
I suspect it's just because it's a beta, and not quite ready for release yet. I swapped in the Windows SSD and am currently ripping it using Leawo. Maybe when MakeMKV is released, it will be ready for prime-time. There's no great rush, for my part, because we mostly watch content on Netflix, Britbox, and Disney+. I just wanted to add the BD version of American Graffiti to our collection. It's the kind of classic we like to own, not stream.
Meanwhile, I now have flatpak available, and I know how to use it, so this was not all a waste of time. That gives me options that I didn't have before, and I appreciate the education that went with it!