In fact, what you said doesn't make much sense, if the switch is connected directly to the MX or not, the operation will be the same, the difference is that you will have fewer jumps if you connect directly to the MX.
Connecting the switch directly to the MX will also not make the network more protected or less protected.
My suggestion is that you use VLANs for network segmentation. Because that way you will be isolating the Broadcast domains not to mention that you can work with different network policies for each of the VLANs.
But remember that security is far beyond the firewall, there are several mechanisms to protect the network that are not much more effective than the firewall itself, just think about it ok?
Topology speaking your network is very simple so it should work fine for you.
As for the issue of limitations regarding the MX84, in fact what may be the limiting factor is the number of clients on the network, as there is a maximum number of clients supported.
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.