The issue I'm facing is that the price points supported by Apple's IAP system are simply too low for my app.
No, that’s not your problem because:
This app allows users to purchase exclusive exhibition spaces to showcase their artwork in prestigious physical galleries.
That sounds to me like the product is delivered outside the app, so it’s not something you can use IAP for.
Your problem is that App Review have failed to understand what your app is doing. (Assuming of course that what your real app does is very similar to this art gallery example.)
There’s not much you can do about this except appeal the rejection and explain the issue in very simple terms. Have you confused the appeal by saying that the price points are not high enough, as you have in this post?
PS why is this an app rather than a website?
PS2 IAP prices up to $10,000 are supposed to be available “on request”.