Can I enhance and resubmit my rejected Swift Student Challenge project with added features?

I'm not a judge if that's who you're looking for. In fact I am also an applicant (who also applied during the last 2 years), but I hope my thoughts will be helpful to you. Don't rely solely on them, though :). Note that I haven't submitted the same idea twice, so I am speculating based on the T&C.

Short answer: I think it depends on what your substantial changes actually are. You're saying they are substantial, so if your app is very different from your previous submission (but shares the same core concept), it might be fine.

As you probably know, the terms say that one of the reasons of disqualification is if "You submit the same app playground that you submitted previously, with minimal or no changes." I think this means you shouldn't submit the same app, with just a few new lines of code & features. So, unless you submit a very similar App Playground (almost the same one), it is permissible.

I think there's two main things you should do:

1. Don't submit an app playground that you submitted previously, with minimal or no changes. Otherwise you might be disqualified.
2. Brainstorm what you could've done better in your submission (better implementation / better idea etc.) to change the outcome of your submission. Even if you don't have the same playground and it is new, it could still be rejected, so try to find out what you could've done better to turn it into a winning project. Do you think your substantial changes might turn this into a winning submission?

now that I have had the opportunity to learn more about Swift and SwiftUI over the past year

Try to showcase that. I would rewrite most of my app and use my new knowledge.

It sounds like you think your idea was great but you could've worked more on its implementation. If that's the case and you truly believe the idea is a good one, I would try to rewrite most of the app and add some substantial changes as you said. I'd think of it like this: how much of the app (both code and feature-wise) is new? If that's above 50%, it should be acceptable, and if it's above 75% then I would consider it "A new playground with some features from my older submission".

Again, I am not a judge, and I haven't tried submitting the same idea twice. It totally depends on your submission.