I'm planning on trying ash at the weekend when I have time, but right now it's still a little nebulous to me. Have you found a boundary where you end up just being constrained and you're just back to normal phoenix dev?
Which is all to say if there's been something I couldn't figure out in Ash I've tended to put it into a regular Elixir module rather than anything Phoenix-specific.
The only exception I can think of is that AshAuthentication doesn't provide support for API keys out of the box, so I have ended up writing my own little plugs for that.
If you are just getting started I'd strongly recommend working through tutorials and/or the Ash book (it's in beta but pretty solid) rather than just diving into a project. It's a lot when you're first wrapping your head around it.