Those RSSI levels appear low for 0.5 miles. Have you made sure you have aligned the SM properly to the AP? I dont know the exact details of your installation but I would recommend you check LINKPlanner first to see what kind of performance you should be getting from this deployment. LINKPlanner can be found here: https://support.cambiumnetworks.com/files/linkplanner/
Now, can you please post a screenshot of the Monitor->Wireless page of both the AP and SM while running the throughput test. The number of wireless packet retransmissions and MCS spread will give us a good idea of why the throughput is low. Also, with an RSSI of -74 and an interference of -86 gives us a C/I of 12 which partially explains the throughput you are getting on the wireless link. Can you please run another Spectrum Analysis at the AP and post the results?
As for the internet bandwidth test, what tool are you using to do this bandwidth test? Is it speedtest.net or something similar?