Stupid question from someone that should know better.

TL:DR; Can I use a single dumb switch for the same layer 2 but separate layer 3 networks?

I spent a few years of my career ostensibly as a network engineer. I have old MCSE, CNA, and other certificates that say I should know the answer to this question so I'm aware I look like an idiot asking but...

I've retired in the last few years from tech and settled down running my own business. It's a campground and WiFi is a crucial service to my customers. I have an outdoor mesh network and firewall that all work great. The firewall has 5 LAN ports. This is not a situation where I'm looking to run VLANs or anything overly complex and expensive. Each port is a seperate NATed private IP segment with DHCP served by the firewall. My question is; if I run all those ports into a single switch (just a flat switch with no user configuration) am I going to create loops or problems with DHCP? This is just a matter of me trying to be cheap and not having a bunch of cheap 5 point gigaswitches stacked all over the place.

It seems to me the layer 3 stuff should eliminate the loops, but I'm worried unnumbered DHCP requests will be routed weirdly or I'll get the dreaded flashing lights of doom with traffic circling endlessly.