I have two feeds established.
For one I've now ticked the "Include HTML Content" and the feed now contains HTML-code. However, it simply contains the whole show-notes as summary. And I remember that was the reason I disabled 'Include HTML Content' months before. It's far too much text in the show-notes to be provided as summary.
From my perspective I expect the show-notes for every episode be visible if you visit the episode using a web-browser. For any podcast-catcher I expect the summary to be taken. Maybe I'm wrong here.
Ah, and yes, I'm already now using some templates for the displaying my episodes on the dedicated episode-page.