To be quite honest, I personally prefer this approach.
Using an L3 Switch as the network core to route all internal traffic and the MX only to control what goes to the Internet.
Some people think it's simpler to leave everything on the MX, but I think I prefer to separate it into layers. The only traffic I would leave on the MX as a gateway would be to the Guest network and DMZ, to isolate communication with my internal network. 😄
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.