HWMP+ Portals

Hi all

I am looking to create a HWMP+ mesh which will have a number of portal sites.
The portal sites are connected by fiber and over the mesh.
My question is should the portal sites be connected by layer 2 or layer 3 over the fiber.
The network will be accessed from another subnet but i don’t know if the portal sites should all have different gateways or the same one.
Or should be portals be the routers as well.

If they are all connected by a layer 2 network then do all devices in the mesh get setup with the same gateway and the traffic will still flow to the closest portal site and then go over fiber to the gateway ?

Thanks

bump