The tl;dr answers to your questions are "a lot" and "we don't really know," but let me elaborate.

Archaeologist Michael Coe has tried to contextualize the Spanish burning of the codices by getting people to imagine if all of European history were reduced to four prayer books from the 1600s. That's a powerful image, but I don't know that it really accurately reflects the damage done. Part of the problem is the classic archaeological mantra that "absence of proof is not proof of absence" -- so we can speculate all we want about what was in all those other burned codices, but we simply can't know exactly what was lost. It could be that all codices were very similar in content and we would have known much more about religious practices, but not much else, or it could be that there was a wide variety of codex types and we could have learned much more about lots of aspects of Maya society. There are a few Spanish accounts that suggest there was a wider variety of codices that included ethnohistorical accounts, but it's hard to know exactly how much to trust any Spanish person knew what they were talking about when it comes to Maya writing.

One thing I can say with certainty is that having all the codices certainly would have changed the archaeological process dramatically. At this point we can read about 95% of Maya hieroglyphs, but getting to this point took decades of work by Maya epigraphers painstakingly piecing together every scrap of evidence. In a world where the codices were never destroyed, it's possible understanding of the script would have never been lost at all and knowledge of it could have been passed down organically from generation to generation. Ironically, it was the notes of Diego de Landa, the Spanish bishop who did the most fervent destruction of codices, that were the eventual key to reading the script, but without the destruction in the first place, we would have avoided a long period of complete lack of understanding about the Maya.

For decades there was a misconception that the ancient Maya were a peaceful group of astrologer scribes who worshiped the passage of time. That era only came to end once we could read the inscriptions at archaeological sites that frequently described military campaigns, human sacrifice, and bloodletting rituals, so any head start on understanding that writing would have completely transformed early Maya archaeology.