Still, it is not difficult to determine someone's racial background by phenotype alone. And adding 10% African or Asian genes to a gene pool of a European population, let us say the French, will change their average appearance for ever.

That's way too much. I'm only an advocate for people that have majority white DNA living with other whites. Far less than 10% of the population in most Western societies is even mixed. If mixed individuals with one white parent were living with whites it'd be ~4% non-white contribution at most.

Africans have around 19% dna of an unknown hominid.

It's more like ~7% archaic human DNA in Africans:

According to a study published in 2020, there are indications that 2% to 19% (or about ≃6.6 and ≃7.0%) of the DNA of four West African populations may have come from an unknown archaic hominin which split from the ancestor of humans and Neanderthals between 360 kya to 1.02 mya.

https://en.wikipedia.org/wiki/Ghost_population#Population_genetics

So I would still say that any mixing is detrimental, as it affects our unique phenotype, causes psychological harm to the individual person of mixed heritage, and infuses our gene pool with inferior genes

As I said earlier I'm advocating for already mixed individuals, so what you consider loss of some phenotypes would've already been done. As for psychological harm, it can be damaging for some mixed individuals but that's not a rule.

Source on the amount of mixed-race individuals with any mental illness:

Zane and his co-investigator, UC Davis psychology graduate student Lauren Berger, found that 34 percent of biracial individuals in a national survey had been diagnosed with a psychological disorder, such as anxiety, depression or substance abuse, versus 17 percent of monoracial individuals.

https://nationalvanguard.org/2018/12/mixed-race-people-have-more-psychological-disorders/

The majority of these people will not have any mental illness and the vast majority likely have none or benign mental illness that is easily treated.