You're correct, Meraki client VPN only supports a single subnet. It is fairly limited in its features.
If you need to restrict some clients from accessing some parts of the network, you will need to do so using group policies.
Create multiple Client VPN for Multiple Network - The Meraki Community
Meraki also supports Cisco Anyconnect VPN, which can provide additional features and profiles, but only one subnet for address pooling. Note that this does however require an additional license.
AnyConnect on the MX Appliance - Cisco Meraki