diy building automation controller and some kind of visualisation platform

Hello.
I have recently committed some building automation. The brain of my system is an arduino mega, on which sits all the logic for output control, input reading, temperature reading via a remote module, additional LED displays. The mega (I'll call it the CPU because it connects to all peripherals via another uC) is connected to the Raspberry. Thanks to it, I have visualisation of parameters and remote control on tablets. I programmed the Raspberry from scratch myself (web server, php, ajax, and the communication layer with Arduino is a python script).
It looks like it looks, there is no shame. On the other hand, any domoticz or openhab looks much better and has a lot more possibilities.

I made some timid attempts to search the Internet for information on how to make my controller communicate with such an assistant via a serial link. I have not been able to find anything interesting. Of course, I am ready to change the transmission protocol or create it from scratch, the thing is to know what to send to the Raspberry and how...

I wouldn't want to get into adding a network module to the arduino and running MQTT.

mysensors gateway - this should help significantly.

And have you considered whether you really need it? I did my automation based on PHP and HTML/JavaScript, and hardware based on ESP8266 and a PC server (also works as a camera recorder, NAS etc) . It doesn't look great, but it works. Over time I stopped getting interested in it because automation is there to be automated. Now I am interested in it when something happens e.g. there is an alarm.

I'm thinking of duplicating the system in two other houses and I'd like to make things a little easier for myself and use a ready-made...