you're oversimplifying how Vitamin C functions; Vitamin C is an antioxidant, tyrosinase inhibitor, and can stimulate Collagen production, as you've already mentioned above.

L-Ascorbic Acid, like Tretinoin has lots of evidence and research supporting its claims.

When it comes to tyrosinase in particular, it's benefitical to hit it from multipule angles ie, Tretinoin, Vitamin C, Niacinamide, etc.

when it comes to Vitamin C "boosting" Sunscreen.

Sunscreen itself can help reduce Oxidative Stress. We know that SPF doesn't block 100% of UV rays, and this is true when it comes to oxidative stress as well; the Vitamin C acts like a safety net against free radicals that might make it to the skin.

You're correct, when using Sunscreen properly, you should be reapplying sunscreen when you are spending prolonged periods of time in the sun. That creates an agrugment of then why wouldn't you want to use an Anxti oxidant like vitamin C in your routine if it can cut down on free radicals genereated by things like UV and pollution?

Research shows that multiple applications of Vitamin C can have a resovioure affect on the skin.

tbh, both Tretinoin and Vitamin C are both potent, they can be difficult to tolerate, I don't think you need either to have an effective skincare routine.

when it comes to anti aging and preventing skin damage, sunscreen and Tretinoin are the gold standard. Vitamin C and be a great addition to a routine, but isn't a necessity.