There are a number of app genres that the App Store reviewers decided to be too saturated. Among those are astrology, horoscope, palm-reading, and similar tell-your-future-from-your-biometric apps. If your app is one of those, you can't publish it on the App Stores. You could try publishing it as a Developer ID macOS application – use Catalyst if it's an iOS app. However there are other causes of "Guideline 4.3 Spam" rejection: Publishing similar apps that differs only in content. Publishing white-label apps under your own Developer Account. A competitor's app looks similar to yours. Search the web for "How to Pass App Store Review: Guideline 4.3 Spam" to find out how to diagnose the problem further as well as some potential solutions.

It sounds like your app may have look-and-feel that is similar to a more established competitor. You might want to review competing apps in the same category or targeting the same audience and see whether any strikes resemblance with your app. Notably the ones that has more downloads (or simply been around much longer than yours). For further information, search the web for "Look-Alike Apps are Considered Spam by App Review" – you should find more pointers on how to tackle this issue.

Depends on how you distribute your application. If it's an App Store app, you would need to use iAP for almost all digital services (one notable exception is currency exchange or money transfers, only applicable to licensed financial institutions). If it is accessed from the web browser, no need to use iAP. However privately-distributed custom apps also do not need to go through iAP. That is regardless of the technologies used to develop the app.

The concern is that people with less bandwidth would be more hesitant to download your app. iOS devices would download the device-specific version whereas the Mac App Store would download the entire binary (all architecture slices).

By now you should just get the M1 Mac Mini.

Only public health organizations are able to deploy contact tracing apps to the App Store. You'll need to get the relevant department in your country's government to apply for an Apple Developer account which in turn would be used to upload the app.