I'd be looking at microPython on an ESP - programming is simple; you get easy access to the GPIO/UART/etc; there are MQTT libraries; and the software & platform

I'd be looking at microPython on an ESP - programming is simple; you get easy access to the GPIO/UART/etc; there are MQTT libraries; and the software & platform will be stable. What's not to like? If you go for the Linux/Raspberry option, the coding will be of similar difficulty (perhaps a fraction easier). But Linux / RaspberryPiOS puts you on the never-ending upgrade and security path. There's not much in the way of attach surface on an ESP, but a Pi needs more thought. Especially as (over time) the little Pi is likely to acquire more complex apps ... a web front-end would be tempting :) ... but then you have to upgrade a webserver, and keep all the libraries up-to-date... Keep it simple.