ᐅ New Build 2014: Issues with High Humidity in the Basement

Hello,
I would like to describe my problem to the experts in this forum and hope someone can finally provide a helpful solution.
I have spent weeks reading extensively online and still don’t know what to do.
From 2013 until the end of 2014, I built my own home with a lot of personal effort. Except for the basement, everything else is perfect!
The house has a full basement. The basement is almost filled up to lawn/patio level. On two sides of the house, the basement still sticks out about 10-20 cm (4-8 inches). The rest will be filled by August at the latest.
Key details:
The basement has a gross floor area of about 100 m2 (1,076 sq ft), divided into several rooms. Three rooms have 1 x 1 m (3 x 3 ft) windows.
A straight staircase leads up to the ground floor through a small hallway (about 6 m2 (65 sq ft)). The basement staircase does not have a separate door because it only opens into this small room. This room has a door to the garage and a door to the ground floor corridor. The temperature in this small hallway is usually between 20-24°C (68-75°F).
Basement floor construction:
Tiles
Screed about 5 cm (2 inches)
PE foil
Bitumen membranes (the floor slab is completely sealed)
24 cm (9.5 inches) concrete slab
10 cm (4 inches) gravel layer
The basement’s exterior walls are built with 36.5 cm (14 inches) Poroton blocks. On the outside, they are first plastered, then coated with a thick bitumen waterproofing membrane, and in front of that, there is 5 cm (2 inches) of perimeter insulation.
No insulation was installed under the floor slab, and the outer walls were insulated only with the 5 cm perimeter insulation because it was intended to be an unheated utility basement.
Now, the major problem:
All exterior walls are now developing mold spots during the summer (I already had issues in spring as well). The rooms have a musty smell.
Most mold occurs at the base of the walls near the floor, probably where it is coldest, but some spots appear higher up on the walls as well. I have tried treating them repeatedly with methylated spirit or similar products.
I have also read many forums about ventilation and similar issues. The humidity level remains between 75-80%. Using a dehumidifier only lowers it temporarily. After a few hours, the high level returns.
I know that ventilating regularly does not help long-term, and running a dehumidifier constantly in every room is not feasible. I have been testing different ventilation methods for weeks.
It only creates effort and does not help at all!
I can definitely rule out moisture coming in from outside through the walls or floor slab. Residual construction moisture is also excluded because I have used professional dehumidifiers and removed buckets of water.
Somehow, moisture always finds its way into the basement and causes everything to rot.
Using additional heaters installed afterward also does not help. The rooms warm up slightly, but the humidity remains the same. Currently, the basement temperature is about 16-18°C (61-64°F).
How can I achieve a low humidity level? Should the exterior walls be insulated from the inside afterward to raise the temperature and prevent condensation? Would this automatically lower humidity if insulation is applied to the walls?
Would it help to install insulation panels around the base of the exterior walls at floor level, about 50 cm (20 inches) high? How thick should the panels be?
I cannot afford to excavate and insulate the outside walls completely again, especially to add thicker insulation—it would be very expensive! Also, the floor slab bottom would still not be insulated.
Unfortunately, I didn’t realize before construction how many problems a cold (only minimally insulated) basement could cause! I cannot store anything in the basement.
I hope someone can help me!
THANK YOU VERY MUCH!