You introduce a really interesting idea in mentioning the OpenRTX project, that’s not something I’ve come across.
I would like to point out however that as far as I know, the only open source hardware that this runs on is the M17 module built by the M17 project. It’s my understanding that the other hardware has been hacked to be able to install open source firmware onto it, but I suspect strongly that this would void your warranty.
Consider for example the situation where a firmware update failed. Unless the firmware on the radio and the firmware you were updating are both supplied by the manufacturer, they’re unlikely to assist you and I doubt that any legal requirements exist to make them.
I’m making this point because that distinction might be important to an end user buying a new radio with the potential of bricking it without manufacturer support.