Excessive spitting up?

My 4-week-old seems to be spitting up more and more, to the point where it's now happening several times per feeding (whether that's during the feeding, immediately after, or two hours later). Sometimes it seems like a pretty big volume too.

Has anyone else had babies that spat up this much? It just seems a bit excessive to me.

He still has tons of wet and poopy diapers (he actually poops CONSTANTLY, which has made it very hard to stay on top of a bad diaper rash that he currently has 😕), and we're going to have his weight checked at his vaccine appointment in a couple days to make sure he's still gaining okay. His pediatrician doesn't seem concerned at this point, but I'm not sure she understands just how much he's spitting up.

We've tried things like burping him more, holding him upright after feeds, and feeding smaller amounts more frequently, but nothing has seemed to help.

Can this be normal? He's fed on breastmilk—I actually pretty much exclusively pump at this point. If it turns out I need to make a significant dietary change because he has an allergy, I honestly might just give up and switch to formula. 🙃

Also, is it concerning that he might not be keeping enough of his vitamin D drops down?