@KimcheeGUN 

I understand. Still, you're saying 3-400mbps is ok? If so, is there a way to remove or alter this "built in QoS" per say? It really feels like it throttles it on purpose because the speed ramps up then stays there pretty constantly. Feels as if it could go higher.. it's understandable this is necessary for many clients but why not give way when there arent that many atm? 
what do you suggest?

  @KimcheeGUN 

Right. I tested the uplink cabling, it passes as gigabit easily. I think a good approach could be reverting (again) the AP to standalone, turning virtually everything off except for 5GHz WiFi and set it as channel 36, 80MHz, high transmission power. That really should rule everything out.

Let me know what else you think I could try!

After thorough testing I suspect this may be a firmware bug. I actually get very good speeds and coverage but only for a few minutes. A few minutes pass by and I get capped speeds at around 300mbps. I don't even need to reboot the AP, I only turn the 5G antenna off then immediately turn it back on and the nice fast speeds are back again for a few minutes. How should I reach out to TP link support? Do they read this?