I went ahead and just spun up some back of the envelope numbers.
For N. America (US+Canada) you have by the most expansive definition from Census data possible (including those who identify as two or more races from US data and combining First Nations, Metis and Inuit in Canada), a Native American/First Nations/Indigenous N. American population of ~11.295 million, in a combined population of 364.6 million, roughly 3.1%.
It doesn't get much better when you add Mexico- depending on the count either the second or third most indigenous country (behind Peru or Bolivia) in the New World. They don't actually gather racial categories, so I went for the estimate offered by an Indigenous Mexican advocacy group of ~25 million Indigenous Mexicans or 19.41%. That gets you 36.31 million in a combined population of 493.5 million, or about 7.36%.
It is a massive change to get to 75%. If we are going ex-Mexico you would need a 25× population increase and including Mexico you would need a 10× population increase in a population which is already one of the most "successful" Indigenous populations in the Western Hemisphere (and home to multiple cradles of civilization in the New World, to boot). That's why I think it is really something that can't be tackled by smaller What-Ifs- the Columbian Exchange was a massive event that occurred and reoccurred and had deadly consequences across vast geographical distances and across centuries. It isn't something where you can tweak some small part of history- you need to change millenia of disease evolution in the Old World or the New World to change it. By 1492 the die is cast. Change the material realities of the New World at the point of contact so that they have a better chance against European disease or give the Europeans something worse (which given what we know of the genesis of disease, would likely require a change in material realities before contact).
(It's interesting too in the Canadian/US example because there has actually been an increase in both Indigenous self-identification and in birth rates, with some places and peoples actually hitting their highest population levels since being decimated by epidemics. Less clear in Mexico where the Indigenous share of the population has actually been declining even as self-identification has increased. Also interesting because if you pull earlier Mexico is even smaller by comparison so the numbers are likely worse).
Is the same true of the Americas as a whole, though? Adding South America and the Caribbean? And is it entirely honest, when this explicitly excludes any and all mixed-race people, in accordance with the USA's traditional 'one-drop rule'? If one counts the Métis people of Canada, for instance, then by the same standard, all of the mixed race Mestizos and Zambos across Hispanic America (along with the Caboclos and Cafuzos in Brazil), could and should also be categorized as 'Native American' by the same token. But none of these groups are, specifically BECAUSE of their larger populations (which, in most Latin-American countries, constitute either outright majorities, pluralities, or at the least large minorities), and their identification, both by themselves and by their historical rulers, as subsets of the European-derived Hispanic or Brazilian (or African-derived 'Afro-Hispanic' or 'Afro-Brazilian') peoplehoods in their culture and ethnicity.
As such, probably the easiest way to do this would be to simply have the Mestizos, Zambos, Caboclos and Cafuzos all identify/be classified as 'Native American' (in a manner akin to the definition of a Métis, in law under the
Métis Settlements Act (MSA), as "a person of Aboriginal ancestry who identifies with Métis history and culture"). Especially if one looks at the
demography of the 'pure/undiluted' Native American population, in the context of the far, far higher 'part-indigenous' population of the Americas. Looking at the Canadian/US examples, there's been an marked increase in indigenous self-identification in the past few years precisely for this reason- because of the emphasis on self-identification, whereas previously it would've been the case that the state would simply decide for them- "Oh, you're not on a native reservation, and we're a European nation, so you're European!"
And the same goes for Bolivia, where 62% of residents over the age of 15 identified as belonging to an Indigenous people, and being 'Native American', when in actuality, both if one went by the 'one-drop rule' and if one included 'part-indigenous' people, Bolivia'd only be the 5th most Native American nation in the Americas, and 3rd in South America (behind Ecuador and Peru by the first criteria, and behind both Ecuador and Paraguay by the second- with Paraguay's population being 96.7% Native or part-Native American, but only 1.7% deemed to be fully 'Native American', since 'Indigenous' is reserved by them exclusively for people who have maintained a separate Indigenous ethnic and cultural identity, language, tribal affiliation, community engagement, etc, and thus explicitly excludes anyone with access to 'Euramerican institutions', such as banks and schools). And looking at
how Paraguay achieved this, couldn't a different approach be to simply have Francia's policy be implemented on a larger scale, by more nations upon declaring independence from their European colonial rulers (which don't subsequently have 75-90% of their male population wiped out, as Paraguay's was in the War of the Triple Alliance)?
Or, given that IOTL, in the mid to late 17th century, local authorities across Spanish America requested, and were granted, royal provisos to categorize their mixed-race descendants as legitimate American-born Spaniards- with succeeding generations, also classified as Spaniards, being granted the same privileges as European-born Spaniards, and thus forming a ruling mixed-race, non-colonial elite- what if they weren't granted these royal provisos? And if instead, only 'pure' European-born Spaniards were granted these ruling privileges? Could we see this POD entrenching the perceptions and categorizations of the Mestizos and such-like as explicitly non-European Native Americans? And see OTL's Latin Americans' gaining their independence by rebelling and overthrowing the colonial European-born elites, and thus consolidating their identification (both by themselves and the colonial Europeans) as Native American, non-European ‘Mestizo Nations’, in a manner akin to Haiti's identification as an 'African Nation'?