"By not having one single url fragment in my OPF files. Anywhere. I've tried a bunch of books. " Please can you explain exactly what you mean by a URL fragment?
"I think you're making a mistake by having your html ToC in the same xhtml file as your Start point. Those reference entries for those two items in your OPF need to point to two separate files. And the file that contains the ToC should contain nothing but the ToC, IMO. "
The title and toc are in one file and referred to for the toc entry, in one guide entry.
The start of the main book is where the start should be, that is in the second file, and it is referred to in the other guide entry. That entry does not have a #start at the end, I just pointed it at the start of the whole file, which I think is what you meant.
So the only thing I'm doing that you feel is unwise is having the title in the same file as the toc. It doesn't sound likely that that is the key thing to change, but who knows.