ᐅ Energy-Efficient Renovation of Older Buildings – Is a Ventilation System Necessary or Not?

Hello everyone,
we are planning to energetically upgrade a house built in 1978.

The goal is to achieve the KfW 85 standard (after consultation and calculations by the energy consultant).

Current situation:
Ground floor: 24cm (9.5 inches) expanded clay with 5cm (2 inches) internal insulation
Basement: 30cm (12 inches) concrete blocks

The exterior walls will be insulated with 10cm (4 inches) material with a thermal conductivity of 0.035 W/(m·K), and the ground floor with 10cm (4 inches) EPS insulation with a thermal conductivity of 0.032 W/(m·K).
The windows are already triple-glazed.
Additionally, a partial extension in timber frame construction will be added to meet the KfW 55 standard.

I have not yet discussed ventilation systems with my current carpenter and planner, with whom I want to carry out the project, as this topic has not come up so far. Since I am currently researching room climate and air conditioning (see: https://www.hausbau-forum.de/threads/Wärmepumpe-zur-raumkuehlung-in-kombination-mit-split-klimaanlage.42180/) and have contacted a company about this, they pointed out—independently of installing an air conditioning system—that I should definitely consider installing a ventilation system.

Does anyone have experience or recommendations on when a ventilation system is useful or even necessary, and when it is not?
What problems might I face if I decide against installing one?

I am generally inclined to prefer a decentralized ventilation system. This might even be retrofitted in the ground floor, which is not directly affected by the renovation (except for the insulation). I see a centralized ventilation system as very complex and demanding...

Best regards and thanks,
Christian