I'd use a Regex or something to make look like this.
<h2 class="ChapLabel" id="ch04_ch_001">Title text</h2>
Chapter heads should be <h
No need for links or spans in a chapter title, even if two lines:
<h2 class="ChapLabel" id="ch04_ch_001">Chapter 4<br/>
Chapter Name Text</h2>
level 1, 2 or 3 depending if more than one book in the ebook (omnibus) and if "Parts" or "Sections" or "Volumes" in any of the books.
If there is a preamble Title and preamble title then they get <p class="whatever", not an <h
Often I'd search </h2>
<p class="reg-body"
and replace </h2>
<p class="no-indent"
Then copy the CSS for .regbody and rename it to .no-indent, then set text-indent: 0;
Then Calibre can easily make a new TOC if the TOC is stupid, and break file at each heading (some ereaders and formats need that to do a page break).