Cover shaft 1.5 x 1.5 concrete like structure

Go to your structural engineer with the sketch. They will immediately see if it works. You might also want to ask if you can simply stick a few rebar into the wall instead of the bearing pockets.

As already said, I am not a structural engineer, otherwise I could have just gone directly to him to tell me what to do. Therefore, yes, the question here in the forum.

A quick question about the alternative concrete

Would it also be possible to install wooden beams there with joist hangers and then clad them with OSB boards and seal them in such a way that the weather and standing water inside don't affect them?

If I stick XPS boards to the inside of the OSB boards and then plaster them with swimming pool plaster and corresponding corner tapes, it should work, right?

With pressure-treated wood, it can last a few years. I wouldn't do that.

But if it is sealed, basically no moisture gets to the wood, right? Roofs, especially of garages, are built the same way, only that the wooden beams are usually not covered.

Active water does not get to the wood.

Then you should ask a wood specialist.