You are correct, especially about personality. Personality is a very very complex multidimensional construct. The only way we can sort of say someone has a specific stable personality is through a statistical mechanism called latent profile analysis. Even then, that is for figuring out general profiles in the population, not the individual.

What we typically do is look at either personality facets or profiles and do experiments to see how they moderate or mediate different effects such as job performance, well being, turnover intentions, etc.

If you want to look at the best scientifically valid personality assessment I'd suggest the HEXACO. There is no bullshit astrology, it's all just psychology and math.