Electric window opening as an alternative to a ventilation system?

As far as I know, one should never do that. If there is a ventilation system, it must run 24/365. The exhaust air duct collects much more than just grease residues. These are pure bacteria and mold breeding grounds.

This is pure scaremongering. There are plenty of reports here from users who have had a central ventilation system for many years, and thanks to affordable filters, after many years of operation they have ducts that look like new. Dirt and such can just as well accumulate in the window frame or in decentralized ventilation. Besides, whatever is not in the cleanable exhaust duct would still be in the rooms.

That is also where the problems in winter come from, when in some cases the humidity drops to 10-20%. Turning it off is simply not an option.

In central systems (for a comparatively small additional cost relative to the total expenses) there is the option to use a heat exchanger with moisture recovery, a so-called enthalpy heat exchanger, which helps control dryness (and also excess humidity during muggy weather!).

My central system also has, among other things, an automatic bypass, i.e., in summer, as soon as it is colder outside than inside, for example at night, the heat exchanger is bypassed and I get pleasant night air into the house. When it gets warmer again, the heat exchanger switches back on and cools the incoming air. There are several adjustable parameters in the control system.