If you just run sphinx-build with the latex builder and then run xelatex or pdflatex on the result you'll get one fully-consistent PDF with everything it it, including fully functional internal hyperlinks. That's what I do for PDF. I can make big documentation packages this way building 2000 page pdfs in a minute or two on a modest laptop.
In the product of the singlehtml builder, you will have the entire document in one single DOM tree. For large documents, even modern browsers on a modern machine will be brought to its knees.
This is a huge document, and having this all rendered naively in one single page will not only be hard to navigate, it will also feel really sluggish if not crash the browser.
Wait: also, how is what you're saying different from the built-in singlehtml builder? https://www.sphinx-doc.org/en/master/usage/builders/index.ht...