Sunscreen question

I know the official answer is to wear the highest most potent SPF all the time, but please be candid.

I don’t have a superpale ginger/nordic complexity but I also don’t get tanned extremely easy like some Greeks or Italians.

I’ve been going out in the sun for a month already and got a nice tan at the moment. I’ve been using SPF 50. My question is, can I use SPF 30 now that I already have a tan, or that is irrelevant? If so, what’s the purpose of these less potent SPFs, are they just for naturally dark-skinned people?