How can the relative word 'what' refer to 'person(s)'?

The fused relative word 'what' generally means "the thing(s) that/which".

But there are some instances where this 'what' seems to refer to 'person(s)' as in these examples:

(1) From a Bustle article titled "https://www.bustle.com/p/julian-castro-on-exploring-a-2020-run-the-possibility-of-becoming-our-first-latino-president-15532402":

Bustle's Alicia Menendez spoke with Castro about his decision to explore running, the stakes of the election, and the possibility of becoming America’s first Latino president.

Alicia Menendez: You are the first of what many anticipate will be about a dozen Democratic hopefuls to officially announce an exploratory committee. Why do it now?

(2) From a CNBC article titled "https://www.cnbc.com/2018/12/23/james-mattis-exit-from-pentagon-threatens-americas-foreign-policy.html":

Mattis was seen as the lone remaining grownup in Trump’s Cabinet, willing to push back against a commander-in-chief who disdains the government’s foreign policy apparatus, and has little use for traditional diplomacy.

“Secretary Mattis represents the last of what we might call the mainstream foreign policy thinkers in the Trump administration,” said Jim Lindsay, who recently co-authored the book The Empty Throne: America’s Abdication of Global Leadership. “His departure is going to shape the balance of advice the president gets. And I think it is also going to change how American foreign policy is viewed overseas.”

In these examples, can 'what' be replaced with 'who' or 'those who' as follows?

You are the first of (those) who many anticipate will be about a dozen Democratic hopefuls to officially announce an exploratory committee.

Secretary Mattis represents the last of (those) who we might call the mainstream foreign policy thinkers in the Trump administration.

If this is possible, which is better? And is there any difference in meaning?