ᐅ House (built in 2019) always too warm from April to October

Hello everyone, I wasn’t sure under which category to post this topic.

We have been living in our house for about four years now; it was built in 2019. The house is built into a slope, with the ground floor containing the living area, kitchen, workspace, and a small bathroom, and the basement level having the bedrooms, building services, and utility room. The attic is not converted.

Now that it’s slowly getting warmer, I notice again, as I do every year:
It’s so warm inside the house. The heating system, underfloor heating (heat pump), is no longer running. We have a thermometer in the cloakroom on the ground floor. This morning it showed 23.4°C (74°F), stuffy and uncomfortable.

Our house has a ventilation system (Wolf), a central ventilation unit. It runs at night and is supposed to bring fresh air into the house or extract air from the rooms as needed.

How can it be that we always have such heat? 23.4°C (74°F) in the hallway?

Of course, I should mention that we get a lot of sun. On the ground floor, in the living area, there are large south-facing windows which we shade when the sun is shining.
In the basement, the terrace door is 2m (6.6 ft) wide and also exposed to the sun. We shade that partially.

We installed the ventilation system back then because it was said to save us from having to open all the windows every morning. Now, between April and October, I basically spend my mornings opening all the windows anyway to get rid of the stuffy and unpleasant warmth inside the house.

Does anyone have similar experiences? How can it be that after a night with 12°C (54°F) outside temperature, the hallway is 23.4°C (74°F) at 7 a.m.?

When I leave the bedroom and walk up the stairs to the ground floor, I can feel the temperature rising with every step :-/

Could this be related to the building services system? The house was built in 2019, with 36cm (14 inch) Porit bricks.

Thanks for your help, best regards
ina