The system I designed now works well, and I learned a lot doing everything from scratch, from the electronics circuits to the web interface. Sometimes it was very fun, sometimes it was very frustrating. But learning this way was always challenging, and rewarding.
However, I think reinventing the wheel is not a good thing. Some parts of SmartHome could work better if they used some proven sources.
For an example, I want to move the server low-level part to Node-RED project. It will add flexibility ad infinitum, without changing anything to the electronics. Writing python SQL requests from serial input is fun but Node-RED could do it better, with simple scripts.
This way, I could put my ressources in what is really fun for me : low level stuff, designing new nodes and functions, and designing a user interface.
On an other side, the central server would be far more useful with some more robust hardware. The Raspberry Pi misses a real SATA connector, this limits the storage capabilities. I don't wan't to rely on SD-card, network drive or USB hard disk. It also misses some computing power to add some multimedia capabilities.
I want to move to the Banana Pi board, it would be perfect for this use. But the board software is still too young for an intensive use. This is a matter of days/weeks as it's improving very quickly.