Which heating system for a multi-family house, heat pump not eligible for subsidies?

Is the hot water storage as planned?

That is the crucial question. With 15 people, you really have to think about hot water supply.

A Bosch buffer storage tank STORA BH 500-6 ERZ connected to a fresh water station

A Bosch buffer storage tank STORA BH 500-6 ERZ connected to a fresh water station

With that, you then get four bathtubs full and afterwards someone still showers warm?

So you get four bathtubs full and then someone still showers with warm water?

Once plausibilized - 2/3 adult persons out of 15 people estimated 30l warm water/person/day = 450l. In the morning there could be warm water consumption peaks, which make up the majority of the usage. The storage tank should preferably not be completely drained. So I would calculate a factor of 1.5 on the daily consumption for the storage tank. This results in a calculated volume of 675l.
I would go for a 750l storage tank, with solar thermal larger, to fully utilize the energy.

Once plausibilized - 2/3 adult persons with 15 persons estimated 30l hot water/person/day = 450l. In the morning, there could be hot water consumption peaks that make up the bulk of the consumption. It is better not to completely drain the storage tank. So I would calculate a factor of 1.5 on the daily consumption for the storage tank. This results in a calculated volume of 675l. I would go for a 750l storage tank, with solar thermal systems larger, to fully utilize the energy.

That’s what I wanted to get at. The storage tank mentioned by the OP is available up to 1,000 liters. At 500 liters, the users will complain quite a bit. With four 180-liter standard bathtubs, you have 400 liters of nicely warm water that goes in en bloc. The first two bathtubs will still be nicely warm; I would only visit the last bathtub with neoprene. Eight showering people more or less at the same time comes to the same result – it’s rather over 500 liters of hot water that then goes through the pipes if you shower extensively (and, for example, many of us know that women with longer hair need a lot of water).

whether people shower more or less at the same times results in the same outcome

and be careful what you install in the apartments. With a powerful rain shower, 30 liters per minute can easily be used.