Ventilation system cellar without windows

The 70% is actually too high and ventilation does nothing in summer. You then cool 30-degree warm air down to 20 degrees, with correspondingly higher relative humidity. Google a humidity calculator and play around with the numbers a bit, then you'll understand what I mean.
If you want to dry the basement walls in summer, you have to work with artificial dehumidification, meaning a construction dryer. I don't understand why you are so stubbornly resisting this now. The result of waiting is completely predictable. You can do that in winter (when ventilation actually dehumidifies), but not in summer (when the opposite rather happens).

Basically then dry EVERY summer or is this mainly about the drying phase at the beginning of the new building?

So is this a "fundamental problem" in summer?

A fundamental problem winter - summer - house drying. In summer, warm air with e.g. 40% humidity is blown into the colder basement, cools down and the humidity rises. In winter, cold air with e.g. 40% humidity is blown into the warmer basement, warms up and the humidity decreases. I would only store on Euro pallets

First of all, make sure the basement dries out. That is a current problem. If you leave it at 70%, you can be pretty sure it will mold, for example, observe the baseboards - that happens quickly. How dry your house is overall will become apparent relatively quickly. It might be fine already in winter, but it could also take longer since summer with its higher humidity has only just begun. You already have measuring devices, I would just check them every day. Our last house dried quickly. And it stayed that way. Nevertheless, in winter, in the rooms where I did laundry, I often turned on the dehumidifier - at >60% my alarm bells just ring.

The fact that you ventilate moisture into the basement in summer is physics and thus a fundamental problem. The insulated basements of today are somewhat better off than the old ones, as the temperature difference is smaller. Whether this generally leads to mold is another question. It depends on how much moisture the basement itself can regulate. Ideally, it can absorb the moisture without any problems and in winter you then ventilate it out again. In the first season, however, you should give the basement a little kick-start and dry it out properly.

It would practically make sense:

Now in summer, ventilate nicely with the ventilation system (of course continuously and always) and additionally work with a construction dehumidifier.

In autumn/winter, possibly carry out the first storage and do not place too close to the walls.

Are there any tips for storing more "delicate" items such as leather, paper, boxes, etc.? Possibly seal airtight? Do you have experience with good "boxes" or shelves that make sense?

Dry laundry only in autumn/winter in the basement. During the current phase, dry outside.

Observe temperatures in the basement as well as humidity in all rooms downstairs and watch out for over 60%.

Regarding the current equipment in the basement:

We have tiled the anteroom, otherwise all 3 other rooms (storage/technology/laundry room) are untiled, or without flooring, as well as logically without baseboards. We have intentionally not done/had this done yet because we want to give the screed time to dry properly.

Again on the question regarding the ventilation system (the instruction by my ventilation installer is still pending).

Does the ventilation system actually recognize the temperatures inside, or does the system regulate automatically, or does the system just let the warm air flow into the rooms at full power all day?

Is this basically the same for all central ventilation systems, or are there manufacturer differences?

We have a system from Tecalor.