Does it appear native? And I keep hearing that even though swing is really old, it has better performance and that it’s used by IntelliJ. Not sure though

Yes (to a reasonable approximation), after calling:

    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
(https://docs.oracle.com/javase%2Ftutorial%2Fuiswing%2F%2F/lo...)

On Mac, the only thing that looks and feels native is the actual native Objective-C or Swift framework.

Everything else has subtle differences, and yes, your users will notice. You may as well just ship a janky Electron app like Teams. But if you want to impress your users with a native feel, it's the Apple toolkits or nothing.

In my case the users were just really happy we solved the problem, I don't think they would care if it ran on minecraft redstone.

Funniest thing about Swing is that if you theme it (because you need to), you can make it look just as out of place (but modern!) as any Electron GUI.