Regarding ETS: with the ETS demo (please correct me) I can only manage 5 devices, right? My system will definitely have more than 50 devices. How is that supposed to work? I don't feel like spending another 1000€ and having to hope that the programmer even hands over the project to me.
Yes, ETS costs money. You can't get around that if you go beyond the number of Lite devices. Fun costs money.
The programmer
must hand over the project to you. However, he will very clearly tell you that you lose any warranty from his side if you touch the project and tinker with something yourself during the warranty period. But that is absolutely legitimate.
This:
I am well aware that I have to invest something, but the whole system should last 20-30 years and run reliably. Besides, I want the security that someone can always help me without me having to throw everything away.
is in complete contradiction to this:
I had plenty of time over the weekend to look into it. I still came across Loxone, which also works with KNX. But to what extent can I work on it myself, is there anyone who has already worked with it?
Loxone is 100% proprietary and no one can tell you how long it will even exist. KNX has been around for over 30 years with several hundred manufacturers. They will not all decide at once to stop doing anything. A single manufacturer might. Otherwise, only the old Loxone Miniserver actually had a KNX interface. It was never certified and is the exact opposite of “reliable.” The new Miniserver no longer has that interface at all. Apart from that, someone has to parameterize your KNX devices for the connection to Loxone.
By the way, the statements apply analogously to MyGekko. Again, something completely proprietary.