Why would he need a better Uair when he already has Upb for 90% of approaches? Basically instant, doesn't need to be airborne for it, works OOS. He already kind of fits your description, sans the projectile, with combinations of armor and Upb. Making him more inclined in that area as a trade off for armor would probably be a nerf. He doesn't need help in stuffing any harder than he already does: you trying to make Fluffy Turtle Zelda over there or what?
Projectile > armor, but more stuffy stuff stuff potential < armor he has atm. If you want to make a character based on that, you'd probably look to remove or tone down Upb before anything else, because it tends to outshine any other thought when the phrase "Stuffed" enters a dude's mind.
Ideally, Bowser wouldn't have a projectile or armor. He would have 1 frame less on jump squat, faster fall speed or air mobility, smoother wavedashing (what a lump of junk he is LOL), etc. In this kind of crazy game though, idk how you balance Bowser without attempting something like armor or keeping Upb as good as it is. Gotta accept the Bowser annoyance in our lives so that he doesn't auto lose to any top tier