Plug a cable into the GX and test you speed from there 1st. That will give you a baseline for wired speed.
You are currently triple NAT with 3 routers. That is not good 😞 You really should be using the Xfinity for modem only, then firewall, then switch / AP. Additionally if you AP's are not cabled/hardwired to your router/switch you could be forcing them to "half" the speed because they are send & receive over wifi. With networking, often, "less is more". Meraki also works best in "full stack" (firewall/switch/AP), it also you to administrate them all from a single interface.
PS: All wifi will interfere with one another. It's radio and susceptible to noise from other radios. Learn about SNR. We often fix large wifi deployment by turning the power down on AP's to reduce interference.