Guys, I am in a very similar challenge.
I need to integrate PAI with MQTT, HA via a serial "tap in" on the alarm keypad using FT232RL FTDI USB To TTL Serial module (5V).
I read that i can basically connect on RX/TX/GND on the same terminals as the keypad connection and run the wire to the...
I missed the first part of your discussion and sketch...
but I find this interesting and sit with similar challenge. I recently added a Ip180 to my SP6000 for connecting to HA via MQTT and PAI. However, learnt that this cannot be done with the IP180, only with a IP150 which is no longer...