I have odd question for you more experienced folks out there. Would it be possible to take a ready to fly drone such as the Phantom 4 or Yuneec Typhoon H and attach an extra flight controller and receiver to the battery that is already on board and use them to control attachments such as servos or LEDs with a separate transmitter (given that it is on a different frequency than what is already on board)?
Something like that is entirely possible but if you only need to control a payload then you probably don’t need to use another flight controller. I have not tried this with the phantom 4/typhoon h but I know the phantom 3 has some pads that you can get 5v/ql12v to power extra gear. Obviously this would void your warranty though.
However in my opinion for this the best platform will be the 3DR solo as it runs open source firmware with an open module bay allowing developers to add their own payloads - https://www.unmannedtechshop.co.uk/3dr-solo-quadcopter/