Is water softening useful?

Can you retrofit something like this yourself?
My plumber wants to sell me the Grünbeck SD 18 for about 50% above the market price (depending on how expensive an installation would be, which is included at his price).
I suspect he won't install a device I provide myself...

Yes, if you don’t have two left hands and the right tools.

Yes, if you don't have two left hands and the appropriate tools.

And maintenance?

Maintenance is not installation. That was not asked. One must also consider the insurance aspect. If something flies off and the water from the main line floods the house with a pressure of 5-10 bar, the damage will be significant and the question arises who will pay for it since the installation was not carried out by a professional company.

Good and important note.

handle the insurance-related aspect.

Especially because self-installation violates the technical connection conditions (TAB) of most municipal utilities, where almost exclusively registered installers are allowed to work. Especially when the municipal utilities are once again searching for the cause of contamination, you are walking on rather thin ice here.

Maintenance is not installation.

Personally, I would trust myself more with maintenance than with installation. Occasionally cleaning the brine lance, that's it.
Of course, this presupposes a proper initial commissioning.