Something like this is definitely possible. It is a big effort, and hard to say if it’s a good idea or not, I imagine it depends a lot on your domains. One difficulty may be the fact that Ash codegen tasks that you can use with igniter are not comprehensive by any means. They are made to produce useful starter points.
However, there are utilities you can all, like Ash.Resource.Igniter.add_new_action etc. to iteratively build up a resource, but those have basically been added over time as I’ve needed them.
The biggest challenge you may face realistically is that ash_json_api is primarily designed to help users create new JSON:API spec-compliant APIs. It is not necessarily designed to make building an API to an arbitrarily structured spec. You can absolutely write arbitrary API endpoints, generally by using generic actions, but you do end up hand-rolling some logic yourself if you do that.
Something worth pointing out is that if there is “just a handful of main routes”, you may not need to bother w/ all the fancy magic.
Not really a cohesive answer for you, but maybe some food for thought 
Selfishly I love the idea of having something designed to reverse engineer API endpoints into an equivalent-ish internal resource structure and then allowing users to write in their own app logic. Sounds like a useful tool 
