No doubt there were differences between Neanderthals, Denisovans, and 'Sapiens' (and I've never understood why differences in brain case size are considered significant), but what I'm mainly concerned with here are definitions, i.e. how much difference is possible within a 'species'.
Brain case size was a metric/shorthand for intelligence and there is a correlation between cognitive abilities and size/ complexity so early hominids ( think Lucy and the Australopithecines ) did not have the same level of complex reasoning ability that Sapiens, Neanderthals and all the other undiscovered relatives undoubtedly did.
They were however different species albeit very closely related whereas obviously all living human beings are members of the same species.
There is evidence of relatively low interbreeding ( I’m going to stick with that as it’s the scientific term, although I share your distaste of its appropriation by those trying to ‘scientifically’ justify their racist nonsense) between Sapiens & Neanderthals shown by the genetic transmission of Neanderthal genes - less than 2% . ‘ We find that observed low levels of Neanderthal ancestry in Eurasians are compatible with a very low rate of interbreeding (<2%), potentially attributable to a very strong avoidance of interspecific matings, a low fitness of hybrids, or both‘.
I think establishing beyond doubt that there have never been any really significant differences between people would help to some extent with the fight against racism.
Yes absolutely agree and in fact would qualify this to extend the notion of personhood to other forms of life and outwards from Homo Sapiens who as you point out are significantly the same but that is of course another debate