Kevin’s right, and that’s the first thing I’d change. I run a plant and we’ve had two contractors bring in homemade boxes like this. Both went back.
Roughly how it works in the UK. Putting electrical equipment on the market falls under the Electrical Equipment (Safety) Regulations 2016, so it needs UKCA or CE marking with your technical file behind it. You’d be the manufacturer, which is not where you want to be for one box. If it’s hard-wired into the plant room, the installation is BS 7671 work for a competent electrician, usually off a fused connection unit.
So keep the only mains part off the shelf. A UKCA/CE-marked DIN-rail PSU (a Mean Well HDR-15-5 or similar) installed by their electrician, or even a marked plug-in adapter. Your box is then 5V only and you can build it however you like.
Gas is the bigger problem. An MQ-4 is not a gas detector in any safety sense. It drifts with humidity, needs burn-in, and no insurer will accept it. Use a certified fixed detector (for natural gas in a boiler room, look for EN 60079-29-1) and wire its volt-free alarm relay to an ESP32 input with a pull-up. The detector does the safety job and shuts the gas valve if it’s specified for that. Your ESP32 reports gas_alarm true/false and the room temperature, and an alarm rule on the profile handles notifications. The boiler lockout relay goes in the same way on a second input.