Rolled turf or sow yourself??

There is no need to dismantle anything. You simply use a different head or install an additional sprinkler. A robotic lawn mower also does not prevent you from mowing yourself. There are many who use the robot during the week and on vacation, and mow with the reel mower on the weekend.

They all have to / want to blow everything up

Unless you have the combination of eco-warrior, know-it-all, and wannabe block warden. They don’t sit in a glass house and are good at throwing stones ;) there are also these plastic cisterns that don’t have to be buried that deep. Can be combined for more capacity as well. You can also sink them yourself with a mini excavator before doing the garden.

We definitely want to keep the lawn area and only create flower beds around it. I had thought that I would lay the pipes before sowing the lawn. Once again, the question: is an automatic irrigation system even worth it if you only water with drinking water? Or don’t you use too much water and unnecessarily spend money on water that way?

Again the question: is an automatic irrigation system worth it at all if you only water with drinking water?
Or do you not consume too much water and spend unnecessary money on water?

You don’t have to turn on the system if operating it is too expensive for you ;)

But then you should also ask yourself the question that has already been asked several times in this thread: Is a water-guzzling English lawn (hopefully doesn’t complain about that term) in full sun the right form of greening? Or isn’t it more clever to let it grow a bit taller and shade it with trees? Or alternatively, create the possibility to use rainwater (but beware: when you need irrigation for the lawn the most, the cistern is probably emptiest)

Whether it is worth it, only you can decide for yourself. I would never dream of using drinking water for lawn irrigation. In dry summers, the lawn turns brown, afterwards it recovers again. That is nature to me. But that is solely my evaluation and only helps you to a limited extent.

We definitely want to keep the lawn area and only create flower beds around it.

That's what I'm saying, at the moment you want it that way and are sure about it. But life is long and people change.