I don’t see why not, as long as the power amp can handle the 20mW input. I would expect you can turn down the power output of the Digi SX but I’m not certain.
Possible but if you wanted to fulfil regulations, I bet a higher power product would cost less than the lower power product combined with a high quality amplifier (both aiming at the same output power).
A high gain antenna might give a similar result. (not knowing what you are using now)
This one is available. But haven’t connected yet. I’ll do it later, when the main work in the field is over. It is possible to install at a height of 15 meters. I want to know how far the reception will be.
When you have a line of sight with this antenna, you will probably have no problem reaching the 10-14 km distance. Make sure you use ultra low-loss cable to the antenna, or mount the F9P to the antenna base. (10 dbi antenna gain could easily be wasted by cable attenuation)