Thanks to both of you. Understanding the trade-offs I think I'll generally stick to EPubs for the moment (I don't use any of the things kepubs are better for, so it is only a general performance improvement that I can apply selectively).
Given the possible downsides with patch support for OTF, I converted my OTF fonts to TTF (using
https://github.com/shermp/ReadifyFont.git), merci!
Thanks for the feedback and knowledge shared!