Then guest is out. I did this for my sister’s bakery and two cafes, all Ubiquiti, so roughly your setup:
Make a new SSID “store-iot” with WPA2 PSK, tag it to its own VLAN (say 30), turn on client isolation so sensors can’t talk to each other, and in the firewall rules allow that VLAN only outbound: DNS, NTP, and TCP 8883 to the Hermetry broker (443 too if anything uses HTTPS). Block VLAN 30 to every other internal network. That’s what the auditor wants to see, and it’s one afternoon per store.
I wouldn’t add a Pi for this. A Pi is worth it when your devices can’t reach the internet themselves (BLE, Modbus, Zigbee) and something has to report for them as a gateway. Your ESP32s already do TLS on their own, a Pi just adds an SD card that can die in the back office.
On the Hermetry side two cheap extras: on the fridge profile set Allowed networks to each store’s public IP, so a stolen token is useless from anywhere else. And check Plain MQTT is off, TLS only on 8883 is the default but your IT guy may have turned it on while testing. Checklist at the bottom of Device transport security.