It’s not like those scientific definitions are particularly consistent or stable, though. In a large part it’s still a matter of convention when it comes to non human species as well

Science, or you, can make up whatever definitions they wish. The question is not if a definition is "true", but what it's useful for.

Sure, you - or scientific authorities, whoever they might be - can declare that there are multiple modern human species. The question is, what are you intending to use that definition for?

That applies to other branches of the tree of life, too. There's not some objective genetic distance measure which says what's a species and what is not. It can vary widely on different parts of the tree. But it's driven by pragmatism - sensible biologists will not waste time arguing whether some mushrooms should be one species or two, if they otherwise agree on the facts.

If you can't say plainly and clearly what the purpose of your delineation is, and it's in your own part of the tree, of course people are going to have their suspicions.