Retaining wall made of masonry panels - how to properly backfill?

The question is, from when exactly is it a "poor drainage" and how do I know if the soil is good enough?

There are different possibilities for that. You might have a soil report with a permeability coefficient (kf value). Medium-grained sand is around 10^-3 there. Alternatively, you can simply try it out. You dig a 40x40cm hole in the subsoil, pour a 10l bucket of water in, and see how long it takes to seep away. Repeat this several times.

Honestly, I had assumed that no drainage pipe would be needed at the base of the L-walls with sandy soil. Although this is regularly required for public projects, I suspect it is rather the exception for private projects.

With our soil, there's no way around proper drainage. That's why I'm also planning with a small dry stone wall since these are basically permeable to water.

I mainly wanted to know what quality the soil for the backfill should have.

It must be compactable and drainable.

(...) I would say that drainage is always required for soil, insist on it but be aware that you have to pay for it too.

Thank you for your advice! I will insist on permeable backfill. Sure, that costs something. What about the drainage pipe? Is it a nice-to-have?

There are different options for that. You might have a soil report with a permeability coefficient (kf-value). Medium-grained sand is around 10^-3. Alternatively, you can simply try it out. You dig a 40x40cm hole in the subsoil, pour a 10l bucket of water in, and see how long it takes to infiltrate. Repeat this several times.

With our soil, there is no way around proper drainage. That’s why I’m also planning a small dry stone wall since they are fundamentally permeable to water.

It must be compactable and drainage-capable.

Excerpts from the infiltration report with kf-value. Dynamic probing 1 and 2 are located near the planned wall site. As a layman, I would say: topsoil, partly strongly silty, rather fine sandy than medium sandy => Special backfilling with coarse sand or gravel is necessary, right?

Yes, backfilling should definitely be done and with the infiltration rate, I would tend to also install a drainage system. The rule of thumb for the separation between water-permeable and impermeable lies at kf = 10^-6, you are a bit worse there.