I now also added support for the RB45, the FRS version.
It is the same hardware but has 22 channels defined.

I have some Retevis RB645 PMR radios, where the only software for programming comes from Retevis and is Windows only.
I have started writing a small tool to read and write the radio: https://github.com/marcoSchr/rb645_programmer
It is still work in progress and only reading the radio and writing the default is currently supported.