How do you plan to handle the dynamic IP address assignment? Isn’t it possible that will change on every RPi reboot? If I were forced to use a limited service provider router like yours (and I assume it’s a router/switch/WAP), I buy my own and network it to the provider’s router. I’d turn off the wireless access point (WAP) on the provider’s router (or just not use it) and use the WAP on my own router. From the provider’s router perspective, my personal router would be the only device.
As for running the RPi’s in parallel, do you mean in a fully redundant mode or do you mean master/slave with different functions? The latter can be done with the MQTT event bus binding. The challenge with the former is that you may need redundant hardware (for example, ZWave dongles, etc.) on each RPi.