After trying the split method I don't think it will improve the reader's experience in the case of this particular book...
Theducks also suggested linking to a single file that is 'the expanded contents'. So I'm now considering creating an H1 TOC for the front of the book, with the last item "Expanded Table Of Contents," linked to a page at the end, which would be the full, expanded TOC.
A reader has no reason to need the expanded version until they've read it anyway, and they need to find a particular thing.
[edited out my questions about how to do it]
However, it's my understanding Amazon does not want a TOC at the end because of 'gaming the system' for the K Unlimited "page reads" ... where if someone has to jump to the end of the book for the TOC, they get more 'page reads' than is accurate. However I also read they will now allow it, but if they think you're gaming, they could take action or penalize you. (I emailed KDP but who knows when they will get back to me.)
EDIT: I managed to do what I wanted. For future novices like me, this is what I did:
1. Generated full TOC with all levels.
2. Copied code into notepad and saved as
Expanded_Contents.xhtml [with UTF-8 encoding chosen in the drop-down (default is ASCI)].
3. Right-click on Text folder in Sigil to add existing files, to add that file.
4. Inside the new Expanded_Contents.xhtml file, removed the <div> code around the title "Table Of Contents" and made it <h1> (if you don't do this, it won't be included in the new TOC you generate.) Also edited to say "Expanded Table Of Contents." And Sigil auto placed it at the end of the epub where I wanted it...
5. Generated new TOC using only H1-H2.
6. Generated new HTML TOC.
7. Generated new TOC (ncx) again using all levels.
So now the H1-H2 only HMTL TOC appears as front matter, but the last link in that TOC is to the "Expanded Table Of Contents" at the end of the book. And the toc.ncx has all levels.
EDIT: For future readers, KDP informed me I cannot copy TOC code to make the 'expanded TOC' to place at the end, due to people"gaming the system" for page-reads for Kindle Unlimited. Instead they require a "reference page" at the end, which I was told must be built from scratch. I clarified this would require setting anchors, then links to those anchors for over 300 chapters/sub-chapters, and they still said (paraphrase) 'I realize it's more work but I wouldn't want you to be flagged and banned from further publishing.' (Note: If you take a 35% royalty and are not enrolled in KU this doesn't apply.)
EDT #2: OK, it got no errors in the online ePub Checker, or in Flight Crew, but Kindle Previewer won't convert it... spins forever. Hit cancel and said it might have errors or is corrupt... so not done yet.
EDIT #3: Turned out the ISBN number went missing from the metadata, and I had previously changed the UUID to be the ISBN, so I assume that was the issue. Put the ISBN back in the metadata and it converted fine.