I know he liked to publicize the exchanges where he got the best of someone, and bury the others, and that he was a far, far cry from a public intellectual. Still, he talked to folks about ideas, and that's something that we should have more of.
That should be something that we strive for, but I fear we'll see it less and less. Who'se going to want to go around and argue with people now?
I guess the steelmanned version of his beliefs would be something like, "racial and sexual minorities are an enemy to the white Americans who own this country; they threaten things we value about our culture and society, and we have no obligation to tolerate or accommodate them if we don't want to."
He spoke out against the Civil Rights act. He said the "Great Replacement" conspiracy theory (that immigration is a deliberate attempt to dilute and ultimately replace the white race) is "not a theory, it's a reality." He said the Levitican prescription to stone gay men is "God's perfect law when it comes to sexual matters." (see https://en.wikipedia.org/wiki/Charlie_Kirk#Social_policy)
Coverage of Kirk's killing has largely skirted around his views, because to describe them at all feels like speaking ill of the dead. If you bring up the fact that Kirk was a loathsome hatemonger, it somewhat tempers your message that political violence is never acceptable
The United States, like many developed nations, is experiencing a fertility crisis: it doesn't produce enough families and resulting children to sustain it's current population.
The US could take steps to address the underlying problems that result in declining fertility for it's current population, but it's unlikely to do so for several reasons that all boil down to political realities where the people that are most incentivized to vote (retired people who earn social security) would probably bear the brunt of the (significant) costs of such solutions. See the idea of "concentrated benefits, diffuse costs".
So instead the US uses immigration to fill the gap left by declining fertility rates (an option not equally available to all developed countries), resulting in young US citizens continuing to struggle to form families, and producing a fraying of the social fabric that such an inability to form families is likely to have on a society.
So you can see why some people would be duped into such a conspiracy theory, which purports to explain what people are seeing with their very eyes.
The great replacement theory is the theory that there is an intentional effort to dilute or replace the capital-W White, meaning the historical English/Scottish/Scotch Irish, population of the US, with immigrants and former slaves, and it usually involves a part that says that it is being done to weaken the country against its international competitors. A third part that is usually involved is that the process is being facilitated by and for the benefit of people like "international bankers", "cosmopolitans", "elites", etc., terms which have an antisemitic history.
To steelman it, you would have to steelman at least the intentional dilution part. Not just to say that it is hard to meet our demand for labor without immigration but that someone is coordinating it. Further, I don't think it has any meaning without the part that says it is being done to weaken the country, which you would have to show that not only would it weaken the country, but that is the intention of these coordinators.
Without that, you just have a demographic argument. If "Whites" do not have many children, and the population would otherwise shrink as a whole, while immigration is needed to satisfy demand for labor, then their proportion will shrink, but it is not "great replacement" without it being intentional/directed.
Not of the conspiracy itself (I'm not interested in that, since the literal version isn't even well agreed upon by most of its believers) but of the observable pressures that make the conspiracy attractive.
I think I could convince an average believer in the great replacement theory (who would be a casual believer that doesn't know many of the specific details you've listed at length of the "official" version) that my restatement of the issue is what they're actually concerned about. In fact, I have had productive conversations with right wingers who express such a casual belief in this theory by telling them what I've written here in the comment you're replying to.