Forking NextJS does sound like a huge hassle to maintain. Having a bunch of separated facade packages with some 1st party additional functionality is how I'd do

Forking NextJS does sound like a huge hassle to maintain. Having a bunch of separated facade packages with some 1st party additional functionality is how I'd do it, too. But also allowing for multiple back-ends increases the breadth of the code base and in places where one package supports multiple back-ends you get increased complexity.