I made an open-source, Arduino-based handheld transmitter (see demo video here ). This handheld transmitter can be used in any robotics application that requires user input (i.e., RC car, robotic arm, etc.), but was originally designed for control of Arduino-based RC airplane flight computers.
It has built-in support for WiFi, Bluetooth, serial radios (i.e. RFD900+, LoRA, etc.), touchscreen control, sound, channel mixing, expo, reversal, etc.
All details, feature descriptions, 3D print files, PCB gerber files, code, and example sketches can be found here.