When building EPublish ( https://frequal.com/epublish/ ), an HTML-to-epub converter, I faced similar hurdles. Trying to keep compatibility with numerous e-readers built with different stacks and varying degrees of EPUB versions is frustrating.

I used EPublish for my first novel, Means and Motive, just published here, DRM-free: https://www.amazon.com/dp/B0GYCZJVGX

So far I haven't heard of compatibility issues, so I think EPublish has hit the sweet spot of EPUB targeting. I agree, however, that it feels like the old days of targeting IE6 on the web. Old readers still exist out there, so we have to aim for the lowest common denominator.