Hey There!
P5.js is an extra layer on top of JS like Processing Core on top of Java, it makes things alot easier. You can learn JavaScript if it’s something that yo7 require or are interested to do. But if P5 meets all your criteria it up to you. P5 is another layer of abstraction which can simplify things but does not necessarily have to be better because there can be things maybe which you would like to make differently then how P5 operates. Learning JS really depends what you are looking for / curiosity !