Equip the rooftop terrace on the 7th floor

His main place of residence is the city center. Now he is at home more often and for longer periods – and thus also more often in the "garden" – the roof terrace is larger than a typical terraced house garden and well planted.

Pot with Red-Leafed Maple,

As far as I know, it doesn't like a lot of wind in full sun...
I'd also keep it maritime for easy care... a few hardy grasses, a beach chair or "lounge shell," and vinegar roses in the pots – they are used to wind and weather and if the roots have enough space, they also survive winter cold.
No matter what you plant, you will have to water consistently because of the wind, even in winter. Although, in a wet Hamburg rainy winter... you don't really need to water much anymore.

is way too cluttered.

Practically, a rain barrel would still be useful, but only if you can tap into a gutter.

That's already a bit very contradictory
I think a 30sqm roof terrace will be used exclusively for fancy stuff, like the jacuzzi. But surely not a rain barrel that saves €5 on watering?! The space for it costs several thousand euros.
I also don't believe the OP wants to create a garden paradise on 30sqm...

Lounge with some kind of fireplace shell thing in the middle. Jacuzzi too. Wood flooring. Sun protection with sails, which are available up to wind strength 7-8, but cost money. A awning is for grandpa, forget that.
Plants you put outside the railing as shown, maybe add a palm somewhere; otherwise keep it rather minimalist. In my opinion, you don't try to recreate a single-family house garden on a 30sqm penthouse roof terrace.

Well, I don’t have that many plants either, but containers that have water reservoirs, since I’m often away on business trips for a few days. When I water all the containers/vessels, I have to fill my watering can at least 10 times. And it’s definitely nicer if you can do that right on the terrace. It could also be an outdoor faucet if you’re not into rain barrels, although rainwater is very good for the plants and there are now stylish options for it that can also be used as decoration.

And, outside the railing .. then everyone should think about how the watering works. It should be practical, otherwise you quickly end up neglecting it.

An outdoor faucet is mandatory, otherwise it will be difficult with the Jacuzzi. The wastewater from it should also be considered.

An outdoor water tap is mandatory, otherwise the jacuzzi will be difficult. Its wastewater should also be considered.

I would also ask about an outdoor water tap; every balcony or platform has a drain.
I'll put it this way:
don’t expect too much: it can be very windy, so it can be unbearable to stay up there.
The beach chair is actually a must, even if young people find it a bit uncool.
Also dune shrubs, sea buckthorn, grasses, and wild rose: just take a look at what Sylt has to offer on the coast.
Do the grilling where the food is eaten: Balconyland by the kitchen.

Where is west?