My custom home automation system
hackaday.io
hackaday.io
I don't think I'd want to live in a place powered by that much handmade, one-of-a-kind tech, but it's really amazing to see something that just doesn't take any shortcuts and does everything according to the designers vision.
It's existed for more than 20 years and maintains compatability with old equipment. In my country basically every new commercial and multi-residence buildings uses it for control of lighting, heating, ventilation, etc.
It's not that cheap, but mainly as the market is commercial use. A presence sensor is around €100 instead of €20 for a "smart home" version, but you are getting much higher quality and guaranteed compatability - however I'd imagine it's cheaper than a full on DIY system like OP, especially if you value your time.
The big advantage of KNX is it is effectively decentralised. A switch and a light in a room communicate directly over the bus wire - so your Home Assistant server going down doesn't stop your kitchen lights working.
You can learn the basics in a couple of weekends and install the physical part yourself, or at least how to instruct an off the shelf electrician to do it. You do need to pay €600 for the software to program it, but there are regular discounts on enthusiast forums.