> So I have to decide if I want to continue learning to work with InDesign or learn how to create epubs in a way that gives me more control over the HTML.
No, this isn’t the question. Most books exists already in a print format and the goal is, to transport those well formatted text to html. So, if your book is already in InDesign, stay in InDesign. But if your text is HTML or Word or in Pages, it is maybe better to work with other programs.
Whatever your decision is: Learn to understand the process and built your own workflow!
One last example: InDesign generates wrong CSS for images > it set width and height attributes and some readers will skew the images. So you need first those readers, second you must have the knowledge, what is going wrong here and how to fix it (maybe in BBEdit).
While it might take 5 hours to see AMs training, it will surely take much longer, to understand the whole process ;-)
Kai