Is the price for the prefabricated concrete garage okay?

No, but it must look like the plan later. I have already posted this in another thread. The garage must be completely clad with wood. A wooden stud frame might be an option, but the garage will be backfilled with earth at the lower part. Whether that will work, I don't know.

Then the tip "Carport" is still there. Having it built as a special feature by the local carpenter is rather common than trying to manage it with precast concrete parts. In addition, the wooden cladding is directly included. But I would tend to steel right away because of the green roof; with wood, the material otherwise becomes very thick and costly.
Earth-covered ... it probably won’t work with wood, right? How about catching it with L-bricks or carport/whatever on top?
In the end, there remains a masonry garage with corresponding costs. Or of course none at all.

I have sent the plans to all possible providers. Now the price is just under 36k, no longer 50k. If it goes further down, everything is good. Building walls will be way too expensive. Also, you lose space inside due to the thicker walls. That's not so great with the narrow driveway. By the way, with masonry it was at 54k.