As this is a DIY project, I did not make any detailed mechanical CAD; only a plan layout for the front plate of the transmitter. This is because part dimensions for example of the LCD screen differ widely between manufacturers and it is not a "one size fits all" thing. I also do not have a 3D printer.
Out of the box, no; there are just too many different and incompatible protocols.
You can modify the firmware and hardware to support any protocol or module you want. There are also a number of DIY projects that address this problem specifically, and interfacing to them wouldn't be difficult.
The RF link is stable enough. Keep in mind that this implementation is not designed for security, and is neither interference nor jamming free.
The range largely depends on the specific module, power amplification setting, quality of your antennas, your environment, etc. From my tests, I easily got more than 2km line of sight with monopole antennas and default LoRa settings, with packet loss of less than 3%. This range is more than necessary to control an RC model (of course you won't be able to see your model at such distance anyway).
Latency varies between 20 to 40ms maximum between flicking a switch and getting the output on the receivers pin. As for servo jitter, none. It is a matter of quality of your servos; I have not experienced any issues with cheap ones like the popular SG90 servos.
It is what I had lying around at the time. I initially planned to use an STM32 but the chip shortage for these
forced me to rethink and revise the design.
I found the Atmega2560 to be performant enough for this application after some clever code optimizations and performance tuning.
In fact it takes less than 15ms on average to compute everything including redrawing the screen.
Voice alerts are not currently supported. For now, an alternative is to use custom notifications with tunes to report events.
I plan on working on this in the near future, but I haven't yet found a compelling reason to add them.
The card should be formatted with the FAT (FAT16 or FAT32) file system. Other file systems are not supported. Also try to keep the number of files on the card minimal.