I agree, I wish there were different levels of tanning. For example, if you only sunbathe for an hour, you just get a very light tan. But if you sunbathe for 5 hours, you get a dark tan. The Sims 4 tanning is too much.
They could potentially update this, and they should be able to, since we did get that 100+ skin tone update, but we'll see if they do...
There's also a mod that has to do with skin tanning and sunburn, where your sim gets sun exposure whenever they're outside in sunny weather, and can even have skin problems because of it. But I don't know if they're still updating it; Iron Seagull did a video on it a couple years ago.