Hi im new to all this audio stuff so im looking for some help.

I drive a 2017 vios J it doesnt have rear speakers as stock so I had some sony coaxials put in. Now im thinking of adding a sub or getting component speakers up front. Would it be worth it? or should i invest in a better headunit first? the stock headunit has one pair of rca so i think it would be fine if i got a sub.