Basement renovation in an old building: Do it yourself or hire someone?

The plates definitely have to be removed; they are corroded and absolutely cannot be unloaded. The sand/clay is just not particularly compacted, it would be like walking on a beach... But I need to dig a bit deeper today. Would gravel and something like paving slabs maybe be an alternative? I can't put anything on the sand. However, I would also be concerned about causing moisture buildup again with a screed there...

I think something like cobblestones wouldn't be wrong. We'd have to take a look. Many old floors look like that, they often paved with bricks back then. They can then "breathe". If it's not solid, but like sand, then I wouldn't want that either. You want to be able to put something down sometimes.

Here is an example photo of a wall of the "wine cellar." It can be seen that there probably once was a horizontal barrier. The material is, of course, totally crumbly and disintegrates immediately. However, the plaster there is still very firm compared to the adjacent room, even though the moisture levels in the wall are actually high.