As I understand it, 802.11i WPA2 mandates exactly AES128.
I believe WPA3 802.11ax mandates AES192.
To get to AES256 you need a solution that supports Suite-B, such as a Cisco WLC solution. Note that Suite-B while still in common use is no longer considered secure for the long term due to quantum computing.
I'm thinking that if you have a specific need for Suite-B that Meraki is probably not the correct choice for you, as you probably have some other requirements as well.
Also note you will need Suite-B compliant WiFi clients.