Foundation/Substrate WPC Terrace

I was considering buying these height-adjustable terrace supports

You must have money!

Is that really unnecessary, or how do you ensure that there is still a reasonably level surface when compacting??

I don't care at all whether the compacted surface is level or not (you can also see that in my picture). Because I use concrete bags, I always get a perfect setup.

Because in terms of price, a solution with concrete slabs is of course much cheaper.

But the ground for the concrete slabs has to be suitable. Or you put them on concrete bags as a base and can level them accordingly. My substructure for the 75 sqm didn't cost 100€. Why still so expensive? I bought screed concrete in bags instead of mixing it myself. The stones were leftovers, partly also scavenged from neighbors. Otherwise, they would end up in the construction waste.

And the freezer bags were filled with fresh screed concrete? No space there with the supported beams? Very nice idea and thank you for the photo.

Exactly, just mix it lean and put it in. Nothing will burst, first, a substructure is not that heavy and second, the bags can withstand a lot.

And you create the necessary slope with the length of the KG pipes or do you just shake the freezer bags a bit until it fits?

And do you create the necessary slope with the length of the KG pipes or do you just jiggle the freezer bags a bit until it fits?

What slope? I always install the terrace without a slope, the rain flows through the joints - a slope is at most interesting for the subsoil so that the water can drain away.
A wooden terrace has the advantage that it can be level.

P.S. You can shape the little bags like modeling clay, however you need them.

And once the bag has hardened, do you also bore through the construction beam?