In a mesh environment I would not recommend disabling the 2.4Ghz radio.
Gateway Selection
Once the AP goes in the mesh mode, The AP scans all channels to collect info from all neighbors. If a valid neighbor (in-network AP or Meraki AP) is found, it goes to that channel. The configured channel has higher precedence if a valid neighbor is found on it. If no valid neighbor is found at all from all channels, it stays on the configured channel.
Based on the scan results, the repeater AP develops a table of all the detected gateways and their corresponding link quality metrics. Additionally, the number of hops are also considered and preference is given to gateways with lower hops. Once the AP hears all the neighboring APs, it finalizes a route based on the link quality and the number of hops.
While it is not possible to select which frequency band should be used for meshing, it is possible to manually adjust channel selections to direct the AP toward a desired behavior. To do this, refer to the article on manually changing channels in a mesh network. If it is desired for two APs to mesh on 5Ghz as opposed to 2.4Ghz, then the APs should both be set to the same 5Ghz channel, but different 2.4Ghz channels. Keep in mind though that a frequency band cannot be allocated specifically for meshing, and both bands will still be available for servicing clients unless the SSID is configured to use the 5Ghz band only.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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