[Quoth]

And really it's not that critical to have every <em>. Much italic or bolds on print is a distraction (sometimes absolutely needed for textbooks and other non-fiction)· Excessive use of <em> is tiring to listen too and negates the impact, so if in doubt use normal or if copying an old paper edition, a minimal version (replace drop caps, small caps and maybe excessive italics added by publisher).

The most important thing is text for all images. Next most is how numbers and such are written as well as the layout of tables. Also for partially sighted and people with difficulty reading don't do egotistical drop caps, illuminated caps, small caps, coloured or grey text (loads of stupid websites out there). Make sure text readable with fall back fonts. Margins, line spacing (height) and font size user changes should work.

Using <em> or <strong> is appropriate in small doses if the author wants it.

Most so-called semantics are about people on the HTML standards exercising power, not about real accessibility. They've done stupid stuff in the past. Also not everything in epub3 or HTML is appropriate for ebook versions of printed books.

Test on small and large screens on eink and phone and tablet. Test without publisher fonts. Test at larger size for partially sighted. Test with basic and decent TTS systems. Test in regular and so-called darkmode.

Communicate with the author and don't guess at intentions of dead people.

Don't use new features because you can. Keep it simple if it's the ebook equivalent of paper. Consider an App if it's not an ebook equivalent of paper print.