This has been suggested before, but the problem is that an ecumenopolis would be able to accommodate hundreds of billions or trillions of inhabitants even on a smaller planet. Starsector doesn't support colonies this size. Arcology worlds feature the high population density and artificial environment of an ecumenopolis without the lore-breaking population requirements of a world city. I've seen other players who want this planet type replace the arcology texture in the mod folder with an ecumenopolis texture.
Regarding a cloning industry, this already exists in this mod! You just have to enable it via the settings file. I don't want to implement the other suggestions as they basically just mimic vanilla industries but presumably with more commodity output.
Regarding the population, you are right, but couldn't the fact that the planet could house a few dozen billions be ignored and just have it behave like another planet perhaps maybe except the enhanced growth properties and or a maximum planet pop of 12? (i dont know about this one since you'd have to mess with the original values of the game and I assume that it aint easy)
the other alternative I could think of are Forge Worlds that behave as specialized industries on one single field, i.e they cant produce a lot of different stuff like convtentional colonies, but depending on its focus, lets say refining, it will be pumping out 3x or 4x more metals than normal colonies, the catch is that their stability is brutally low.
I also have several ideas for buildings and or unique resources if you'd like to hear them out so I can write them down.
