I guess the starter might be to build /e/ for a supported device, then build /e/ for a device that has say unofficial support from Lineage OS. Then you could be in a poition to build for a totally unsupported device. 
Or go down the more experimental route and look at a treble enabled phone and a gsi build. (You will know already, treble enabled phones loosely started with phones delivered with Oreo) Getting Started on /e/OS.