I think there’s two official ways of going about this:
1. Publish an alternative app marketplace in the European Union under Apple’s draconian rules. https://developer.apple.com/documentation/appdistribution/creating-an-alternative-app-marketplace/
2. Make @Zapstore also a web client and have the download links go to the official App Store or TestFlight. 

 Yeah, been looking at the altstore approach (essentially self-signing apps). Far from ideal but it would work

And an alternative EU app store, later down the line.