What you are doing makes sense but it doesn’t explain why you making an HTTP call to a PHP program to save the data to a MySQL database when OH could save that data to MySQL directly, even if you have multiple OH instances saving to the same database.
Or instead of an HTTP driven PHP program, you can have OH publish all the events over MQTT and have your PHP program, or whatever you choose, subscribe to those topics and save the data based on that. This is a common approach for integrating OH with external home automation systems like NodeRed. It isn’t just used to link up two or more OH instances together.
In both cases, you can skip the Rule entirely.
Another approach you could take is to just have the one instance of OH and have it connect to your Pi/Controller over IP.
I’m not entirely certain how well the Zwave binding will be able to scale controllers though so that is something to watch out for. I’m sure two or three controllers has been invisioned, I doubt 100 was taken under consideration.
There are a lot of ways to approach this sort of problem and I’m calling into question your approach. I’m sure there are other considerations at play. But it still seems that there are ways to do this a little more natively to OH itself.