Hi!
I am Pranav.
I can design the electronics circuit for you to control the 20 elements. 2 button concept is easily doable, and IMO you can get done with a controller board for < 10 USD in parts, for one-offs.
I can also make this into a mass production-able controller board, if you'd be interested in that.
I am a professional hardware developer. I can make the PCB for you, get it fabricated, assembled, programmed and delivered to you ready to be used.
Let me know of your thoughts.
One thing I was unsure of was the 8mm*8mm*8mm heating element boxes. Could you maybe sketch the idea for me to understand, or maybe send a photograph?
Thanks in advance!