How to upgrade a smart home as much as possible?

That is not entirely true for Homematic. Most systems also run without central control (lighting, heating, roller shutter links). That means even without a central unit, you can operate all devices, or the time programs are stored locally on the devices, for example on the thermostats. Automatic reactions to certain events naturally no longer work without a central unit. However, I only had a single failure of the CCU in 3 years (because I overloaded it through unfavorable programming). Otherwise, it sometimes runs a bit slower but absolutely stable. The effort in programming should certainly not be underestimated, but that probably applies to most systems. On the other hand, it is extremely flexible and combinable with many other systems (e.g. FS20, Philips, Hue, Sonos, all devices controllable via IP, etc.).

However, I must admit that I do not really know Z-Z-Wave well, as it was still in its infancy and very limited 3 years ago when I decided on HM. That might be significantly better today. For me, having a feedback channel is absolutely important, because I don’t want to come back from vacation and find that heating and lights have been on continuously because signals did not get through. This applies even more when controlling security-relevant things (front door, garage door, window openers, roller shutters, awning control).

KNX is certainly the deluxe solution, unfortunately only conditionally suitable for retrofitting and ultimately also the most expensive system. If I were building new, it would definitely be the method of choice, but only because I know exactly what I want/would want to control with it and that the corresponding benefit is there. Anyone who wants to first test how high the added value is should start with a modular system.

and the best modular system is:

"drumroll"

exactly! KNX...

but only conditionally suitable for retrofitting. For this reason, I recommended z-Z-Wave, as it is well suited for retrofitting and can do many things that are otherwise only possible with KNX

If I were building new, that would definitely be the method of choice, but only because I know exactly what I want/would control with it and that the corresponding benefit is there.

May I ask which application areas you currently use and would use again. I am currently dealing with the topic myself and am trying to find out which areas I find useful for us.

KNX is ultimately also the most expensive system.

that is not true... with an appropriate number of devices to be controlled and the desired functions, KNX even becomes more affordable...

it is only more expensive if you want to dim three lamps and operate five roller shutters

Ultimately, all these solutions have one problem -> they are centrally controlled. Meaning: If the central intelligence fails, nothing works anymore.

This is not quite correct at first, although I understand what you are getting at. If the central unit fails, I can, as simple examples, still switch my Philips lamps on/off worldwide via the Hue app or access the surveillance camera via web access.