approaching with your back should be your main approach IMO. You have bair, airdodge behind them, DI away to reset, paralyzer B reversal away, jump away etc. Know your bair frame advantage so you know if your opponent can punish it (most characters cannot).
After that, nair is also a decent approach. It's disjointed which is a very good asset, but its frame advantage isn't as good as bair. It combos if you do land it. However, usually in my experience, your SH aerial approaches are too slow to hit someone by themselves. You're looking to read what they do after they shield the aerial usually.
So for example if I see my opponent likes to shield grab a lot, and their name isn't DDD or olimar, I know that if I space a nair well, they're likely to try and shield grab, and will whiff. So I'm going to nair their shield and dsmash after. There are plenty of examples like this, but frame data is how you can understand what should or shouldn't work.
I also find dash mixups are somewhat effective. Out of a dash, your main options are side b, nB, jump away, jump over their head, roll behind, shield, spot dodge, dash attack, and grab. You just have to figure out what your opponent is doing to stop your attacks from hitting them, and then choose options that either beat that option or push your opponent to use other options