Laying paving stones with a 2cm gap like lawn joint paving?

Cobblestones interlock and support each other, joint gravel further reinforces this. If you lay the stones loosely and step on one stone, it twists and you get a bumpy surface with loose stones

Yes,
real paving stones with "built-in" spacers are used;
loose, placed spacers can also "move" or sink over time in the gravel bed.

Does anyone have experience with greening the joints, does it work as hoped?
So that only the desired and sown mixture grows, and unwanted weeds (dandelions, ...) do not quickly take over?
It should and must not become too time-consuming to maintain, it is 90 sqm and completely removing weeds twice a year :-(

It should and must not become too time-consuming to maintain, it's 90sqm and completely removing weeds twice a year :-(

That's nature... With me, you'd probably have to weed every 2 months – just because of the work on the fields in the surrounding area so much "dirt" gets onto the property.

real paving stones with "built-in" spacers are used;
loose, inserted spacers can also "wander" or sink into the gravel bed over time.
Does anyone have experience with greening the joints, does it work as hoped?
So that only the desired and sown mixture grows, and unwanted weeds (dandelion, ...) do not quickly take over?

You can forget about a "members only" planting of the joints, as a rule the unplanned nature dominates the planned nature, otherwise the landscape would be full of hedges.
You could try setting the stones in mortar spots, which you apply with mortar in a silicone syringe like decorating a cake. Because of the processing speed, this might require preparing small portions and mixing them very fresh. Or you make a substructure from paddock grids in the sand layer under the stones. Just an idea, I have not tried it myself.

There are herbicides for lawns. Ecologically, little happens anyway in the 2cm joints and why you want to more or less seal 90m² this way is beyond me... but then the bit of chemicals no longer matters.

Has anyone had experience with greening the joints, does it work as hoped?

We have applied a special sedum mix for the joints. Everything was applied according to instructions, but it takes time... since June we have been waiting for it to turn green and only gradually something is happening. You can clearly see what is weed and what is sedum, and the weeds are absolutely kept to a minimum. Here and there I pull something out, but contrary to our expectations very, very little weed grows. In our joints, however, there is only this sedum stuff, no soil. Oh yes, around us there are also meadows and fields, agriculture. Unfortunately, this is also very noticeable in the garden; what shouldn’t grow there grows. However, it’s different with the joints.