FWIW, I helped my mom grow a big Natal Plum bonsai back in the 80's and 90's. It was a pretty large developed tree imported by a greenhouse owning friend from Hong Kong, trunk was four inches diameter at soil level. It was not fussy about soil, but it was fussy about having its roots messed with. I repotted it very infrequently, like every five years or so. It sulked for months after a root prune. The tree was brought inside during the winter and set on the floor next to the patio doors. The doors had top to bottom windows, so it got bright, but indirect light from Nov. to Feb., then back outside.
My mom grew it in full Texas sun for decades. She put it on her back patio so it got about eight hours of full sun every day from Feb. to Novenmber. She put in in a shallow baking pan with water up to the bottom third of the bonsai pot. I warned her about root rot, but the damn thing grew like mad for years, bloomed and produced fruit regularly in the summer. Squirrels finally killed it by chewing the trunk, girdling it.
The pan filled with water probably won't work in Pennsylvania, as the plant isn't going to be growing as strongly--it's a subtropical shrub used in hedging in places like South Africa and Southern California. What will help is getting the tree as much full sun as possible, for as long as possible. Wouldn't worry about very hot days. Your Natal Plum can take it and probably love it. Pennsylvania sun ain't Texas sun...