So I am go to have Zss as a secondary and I don’t know why you would use up smash over uptilt. Uptilt does more damage. Why would you use upsmash for combos.
Uptilt feels way more unreliable then upsmash , granted both of them can leave you open.
You have to just use really good judgement on which should be used.
A few examples uptilt has a tendency to let people di out of the 2nd hit I've been punished for this at low/mid %. However I have also gotten of like 3 uptilts in a row on characters that typically try and challenge you as they land(people like kkr or ddd heavies/fast fallers in general), uptilt >party>uptilt ect.
Upsmash can be very good but it's hit box feels inconsistent. Had a dk just fallout( or possibly di out of first few hits) and dong punch me.
Upsmash can almost platform frame trap people of they shield it thanks to shield drop frames. Literally stolen stocks with things like shield pressure upsmash -> sh bair clipping them as they are dropping shield.
Also note both moves just generally can miss if the opponent is to low to the ground, buried a Pikachu on a platform but couldn't hit him with upsmash and cost me a game.
All in all you have to just pay very close attention to where they will be