Why set up the link between the link between the Catalyst and the MX as a trunk? While you can do this, it doesn't add much, as you can't operate multiple SVIs over that link. I'd just go for an access setup in your chosen VLAN, without any tagging. Hard to say if the MX is appropriate as a purely internal firewall, without knowing exactly what you're wanting from it, though it is certainly true to say this is a fairly rare deployment scenario for an MX. Remember that, by default, traffic between different VLANs is permitted by an MX - you'd need to configure specific deny rules. You might also want to look into using no-NAT on the WAN interface: https://documentation.meraki.com/MX/Networks_and_Routing/NAT_Exceptions-No_NAT_on_MX_Security_Appliances
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