The major problem with your post (and with eugenics) is that you assume desirable genetics are good genetics. In reality, there is no such thing as “good” genetics. Either you survive or you don’t, a person with many ailments can live longer and have more children than a conventionally fit person, because we live in a dynamic environment with volcanoes and staircases and murderers. The best genetics are a wide variety of genetics, because the environment is ever changing and what is good today may not be good in 10000 years, and if you breed out the “bad” genetics, you won’t have the tools available to deal with the changed environment.
Any kind of selection of partners should happen as unconsciously as possible. This is partly why most people fall in love with strangers and don’t select partners with the exact optimal genes. There is no dating app that matches you with your exactly optimal genetic partner (don’t get any ideas if you live in silicon valley), people don’t want that and it speaks to our innate desire to select a variety of genes rather than the “best” genes. Even though genes that happen to be good in the current environment tend to be more selected for, this happens naturally and unconsciously which is what separates it from eugenics. Eugenics always involves some authority deciding which genetics are good and bad and then organising social structures to enforce those categories. This is obviously wrong.
You can only numerically compare things that are linearly ordered. 2 is obviously bigger than 1. But you cannot say which of two lists of random numbers is bigger than the other, you would have to define your own set of comparison rules out of possibly infinite rules to compare them and say which is bigger.
Suppose you have 2 apples, one is bigger and has more calories, but the other has higher nutrient density. How do you declare which is the better apple? At best the answer is contextual and at worst it’s impossible to solve. You cannot recommend someone eat one apple over another without making qualitative decisions about their health. Even if you tried to put a number on their health, the problem would recursively cascade down to infinity. They may need more energy to get through the day and so should go for the bigger apple, but why prefer short-term energy over long-term nutritional health? Why prefer the short term over the long term in general? These are necessarily qualitative decisions that utilitarianism cannot solve, because you mathematically cannot decide between non-linearly ordered objects without forming some subjective, qualitative philosophy first.
So when we say ‘you can’t put a number on everything’, it isn’t just a platitude, it’s a fact of the universe, and denying that is like denying gravity.