Well if your contemplating writing your own RC system, be aware it can get quite complex, dealing with all the various TX protocols, telemetry and flight controllers.
There are not that many open source systems available, which is an indication that its not an easy bit of programming.
As I am sure you are aware there have been open source long range RC systems, capable of way in excess of 5km, available for several years. They use modules such as the very low cost RFM22\SI4432 and are available ready built or as PCBs. So there is no particular reason to use the LoRa devices.