I've linked it before, and its not Meraki, however if you ignore the fact that its a document from Aruba, and just look at the math, it basically applies to any vendor. This is a good read for your situation, but basically what @JohnD said, use directionals, channel-reuse is key, disable most if not all 2.4, try to leverage rx-sop, reign in your autoRF settings (power/channel) so they are not all over the place. https://community.arubanetworks.com/aruba/attachments/aruba/Aruba-VRDs/54/4/Aruba_VHD_VRD_Engineering_Configuration_Guide.pdf
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