Which smart home system to retrofit?

Have you already started your electrical planning or do you know your need for additional sockets, light outlets, etc.? The developer's electrician will then invoice the additional services separately with you, provided the developer agrees. It is also the question whether the electrician is interested in implementing the additional service ("special planning compared to other developer homeowners") accordingly. Example: Our electrician only received a small time window from the main contractor for our construction site. I was then left with my pants down: My wishes could or would not be fulfilled!

Maybe you can have something like this included in the contract: "The client requests an electrical installation with star-shaped wiring from consumers, switches, etc. to the home technology room, as opposed to the conventional electrical installation." (like this or something similar).

Unfortunately not

We have roughly already started the electrical planning for ourselves

unfortunately we cannot include anything in the contract :/
we have to hope that our wishes can be fulfilled at an acceptable price.

For the following special requests, we already have the price.
Instead of 2 double doors, a lift-and-slide door 5300€ (KFW 55)
Shower niche 30x30 cm 550€
Underfloor heating throughout the entire basement instead of 3 low-temperature radiators 2500€
Per single socket 50€ double 80€

So that we get a sense of the pricing policy from the developer

What do I have to ask the electrician to do?

just star-shaped wiring? or should we take the whole smart home system completely right away?

What do you think about Homematic IP? Sorry but as I just see I have no idea about smart home.

I think Homematic IP would indeed be a pretty good choice for you – even if not the cheapest. If you take the Access Point as the base, you can control all components with it as well as set up logics and even get an app handed to you. The Shellys are also great, but you definitely need an additional component on top to hold everything together. You can google iobroker, openhab, and home assistant and see if you feel confident to "build" something with it. Price difference for a roller shutter, for example: HM IP Broll: 60 euros Shelly 2.5: 20 euros Surely there are other dependencies, but your goal, budget, IT affinity are not yet so clear.

or should we just take the whole smart home system completely?

First, you have to clarify with the house electrician what would even be possible. If the electrician doesn't want to cooperate and/or charges exorbitant prices for outdated technology, then you have no choice but to bite the bullet and deal with retrofit options. However, you should try to include as much as possible during construction. P.S. The standard wiring essentially costs the same 10-15K in the end, but no one tells the homebuyer because they pay a flat rate. One smart home, please... it’s like one car, please. Diesel, petrol, electric or hybrid? Sedan, station wagon or coupe? And anyway... which brand? There are way too many options and way too many paths.

As a developer, you only have the options he offers you anyway; you are not the client.

The electrician should install at least deep flush-mounted boxes and there should be neutral conductors everywhere. Then you can bypass the switches/buttons with wireless actuators and make them smart (prepare).