Not sure that you can get what you expect without purchasing solid doors and If you expect a smooth finish, that is usually a door with a special facing to receive a sprayed finish.

Appreciate the input but would really like a specific recommendation of a door manufacturer that won't break the bank on this design element : Flat slab door with hidden hinges. And a solid core is ok but I've been told that a stave core door works fine for hidden hinges.

Adam, the problem is not the stave core door, your problem is the facing. Just not sure a company would bother to put a smooth facing on a hollow core door. You'll need to consult with a lumber yard in your region that will do special order items.

Maybe this primed door.

https://www.homedepot.com/p/Masonite-30-in-x-80-in-Smooth-Flush-Hardboard-Solid-Core-Primed-Composite-Interior-Door-Slab-08200/100092843

Here is the problem. There are so many super cheap / garbage flat panel doors for sale that most door manufactures have stopped trying to compete. They gave up the market. You will now have to pay for a custom door and its associated cost. Also, you could look at commercial doors but they are not the standard residential 1 3/8" thickness.