Experiences with brine heat pump

Correct.
Plus the fear meter of the builder (or the drilling company )

I also have a brine-water heat pump without modulation and have had similar experiences. About 4-5 cycles per day in winter, without heating operation in summer there are usually 2 cycles (hot water, 180L integrated tank). This results in about 1250 cycles per year. In comparison, the durability is about 100,000 compressor starts. Expected lifespan of the compressor therefore 80 years.
Wear is therefore in my opinion no argument in favor of modulating brine-water heat pumps. That’s why I didn’t choose such a model, it would have cost four digits more. I took a maximally dumb brine-water heat pump

The main advantage is the volume, as the modulating one often runs quieter. Otherwise, I also believe it makes less of a difference.

Although your compressor starts for non-modulating heat pumps are already very, very few. Many never get close to that. Many have between 5,000 and 10,000 compressor starts per year. Then durability becomes an issue.

Do you maybe also have to take into account whether you are using the "cooling" function of the brine-water heat pump? And then maybe a modulating one makes more sense again?

In summer, the compressor only has to run to heat the hot water. But also for "cooling" if I send my 20 °C through the underfloor heating pipes and pipes in the concrete ceiling?