OK, so my scope may have exploded a bit. I am now working on:

• Zone/output types and attributes
• Zone expander (RS485) support (Arduino Nano)
• RS232 coms with power monitor (Arduino Nano)
• Alarm types: fault, tamper, water, intruder, panic, fire (with escalation)
• Ready LED with multiple flash patterns
• Many other things…

Using 21% RAM and 6.2% flash on the Arduino Mega.

Source here: https://forge.cavelab.dev/diy-alarm/core