Hello Jonah
As you suggested, Cisco Meraki access points are a special case. They are not strictly speaking “lightweight” since they still have a level of intelligence higher than real lightweight APs. They are indeed somewhere between lightweight and autonomous APs. This is why they belong to the Cloud-Based AP Architecture category.
Only control plane traffic is exchanged between the AP and the cloud controller. All data traffic remains on the local network. For multiple SSIDs, it is possible to tag the traffic of each SSID with the appropriate VLAN ID, and configure those VLANs on the switchport for segregation of the network segments. More info on that can be found here:
I hope this has been helpful!
Laz