Is a rainwater cistern worthwhile for garden irrigation?

A rain barrel would have been an alternative (in column shape) about 500-600L

Since the rain barrels naturally require a lot of space above.

Which cistern would you even install underground?

Is a plastic one sufficient?
Flat tank is probably best for retrofitting
How big should the cistern be?

Which cistern would you even install in the ground?

The question is a bit too general. Therefore, a general answer: The one that best meets my needs...

Is one made of plastic sufficient?

Yes, if it does not need to be drive-over capable or located close to structural facilities.

A flat tank is probably best for retrofitting

It depends; you don't have to dig as deep (we had to dig a bit over 3m), but it also requires a lot of space in terms of surface area. So nothing deep-rooted can be planted above it, and you should avoid building anything over it, even if it's just a shed. Later planned frost-free pipes might also not be easily laid there.

How large should the cistern be?

There are calculators for that, just google it. Compared to forum statements here, they are rather conservative (meaning, forum users believe you need much more).

9 a.m. to 7 p.m. are also bad times for watering.

You are right. Now the times are also okay. In June and July, it was still forbidden from 8 a.m. to 8 p.m. And that meant getting up at 6 a.m. and not going to bed before 10 p.m. if you wanted to manage the watering reasonably well.

Now I have to "step in" here. I always recommend that instead of just asking questions here (almost pointlessly), you should know which questions you are asking for what purpose.

We don't know your water requirement for the garden at all. Garden or lawn? 1 cubic meter or 5 or 10? Installation is easy or also difficult etc. pp.

I would be more interested now in the installed floors and the ordered furniture. Thank you.

You are right. Now the times are also okay. In June and July it was still forbidden from 8 a.m. to 8 p.m. And that meant getting up at 6 a.m. and not going to bed before 10 p.m. if you wanted to manage the watering somehow.

These are my times, it can't be done otherwise.
The purpose of withdrawal times doesn't fully make sense to me. It may relieve peak times, but probably not reduce water usage. Especially those who water automatically just reprogram and that's it.

Um, yes, those who water during the day water more in total because most of it evaporates and the plants don’t benefit from it. Basically, makes sense, right?