Here’s a good thought exercise.
Without specifically mentioning the number of people you see, the number of buildings you see, the specific size of those buildings, the specific size (in square miles/kilometers) describe a city in such a way that we could reliably tell how many people lived there. For bonus points do so in such a way that we could distinguish between say, one city with 1,000,000 people and a city with 5,000,000. Or between one city with 15,000 people and one city with 60,000 people.
The truth of the matter is that is an insanely difficult task. When de Vaca came to a village he often did not describe how large it was. He would occasionally mention numbers, but usually did not. He would occasionally mention number of houses, but usually did not. When he just spoke of villages generically he did often did not mention how many houses were there or how many people. Meaning it could have been 10 people or 100. When they fought large groups of natives, he often did not tell how many natives were attacking. When he traveled for extensive periods of time and spoke of seeing lots of inhabited land, he often did not mention the relative density.
And finally, he cut a path from Florida to Mexico, stayed for some 6-7 years as a slave in one region, and probably did not visit many settlements that he traveled very closely to, so even if he had accurately told the size of every single village he visited we still would not have an easy time getting an accurate count based on de Vaca’s observations.
What is interesting to note is that the region de Vaca traveled through could have had 5 million people and it would not conflict with his tales. Five million people spread from Florida to Mexico is not very many, and would look very sparse. Even 10 million would.
You realize Florida today has 20 million people? I can take you to parts of Florida that are extremely sparsely populated, I could walk you 100 miles in around Florida in a path in which you would see few people. Is that evidence Florida has a low population? Of course not.
At the same time, I wouldn’t suggest we accept that Florida has 20 million people just based on walking around Miami and Orlando. I would instead say that such voyages are not an accurate way to come about population estimates. We should instead look at the cities, how many streets are there, how many buildings, you can start to make estimates based on that. (Of course in the modern world we just send out census workers.) What’s interesting is the “Census at contact” premise you’ve started with is essentially a giant straw man. Who has ever claimed there was a census at contact? The work of all professionals I have ever seen in this field instead focuses on archaeological data, conclusions based on long accepted methods of estimating for example how many people would live in a village of x size, how many people would require farmland of x area.