basically we don't need a lot, mostly switch fonts/text sizes/images (generated barcodes, logos) and of course pdf/a-3(a/b/u) for invoice. so apis that translate the html into the layouting of pdf is the bigger problem.
[0]: https://github.com/AngleSharp/AngleSharp [1]: https://html-agility-pack.net/
Yeah HTML to PDF is a tricky one, presumably wkhtmltopdf/pechkin doesn't work out because of licensing / interop issues? Other than that the only other one I'm aware of is Aspose which is expensive as you say.
Images (along with font subsetting and fixing the gzip implementation) are the next thing I plan to implement so it's helpful to know its a real requirement.
btw. itext is a really great library, unfortunatly itext has a problematic licensing and I tought they were jocking after they gave me prices.