I have noticed some complexed loanwords in Ukrainian from German via Polish like the word for taste “smak”. Is it just slight influence that Polish had on Ukrainian or was it related to assimilation or superior/inferior battle? Are Polish loanwords the reason why Ukrainian differs so much from Russian?
The reason why two languages differ is always complex, there's no one single factor, it's the combination of the factors. You can look at it as you've formulated, or you can look at it like Russian was hugely influenced by Old Bulgarian, the factor which wasn't experienced by Polish and Ukrainian that heavily, so in fact (I'd say) it's rather Russian under Old Bulgarian influence that digressed from Ukrainian than Ukrainian under the Polish influence that digressed from Russian.
Don't forget, Russian has tons of borrowings from Polish, too, and also German borrowings via Polish, and смак is also a valid Russian word.