Foam glass gravel yes/no?

Everything is a matter of belief... I didn't do it and don't have a bad feeling about it. Why? Because I also spoke with former energy consultants and all of them were of the opinion that it serves more as computational proof than practical benefit.

You won't get a clear answer here... I also have strip foundations - of course protected against frost.
Underneath, gravel and a thick dirt-proof membrane that does not rot after 5 years. Membrane against moisture and insulation inside with 12cm EPS without a basement in my case... but the pipes are protected separately again. Electrical conduits are not specially protected because they already insulate very well themselves (see Fraunhofer study).

You have thermal bridges in both variants... that cannot be avoided, only reduced.

That's it.

How exactly did your build start from the bottom?

I opened my own thread called "Planning" Insulating the base plate. Please reply to me there, so I don't keep asking the same questions in 2-3 threads at the same time and annoy people.

We decided on the exterior option, among other things because of moisture protection. Condensation always forms where it is cold, and in the case of interior insulation, that is the inside of the floor slab, so to put it bluntly, you have condensation under the insulation. Additionally, there is a thermal bridge in the area of the exterior walls going downward, and thus cold corners where condensation can potentially form. From my point of view, therefore, the exterior insulation, which completely surrounds the floor slab and transitions into the perimeter insulation of the exterior walls, is the better solution.

And regarding the compressive strength of XPS: XPS 3035 CS has an allowable compressive stress under permanent load of 13 tons per m², so a house with a 100 m² footprint could weigh 1300 tons. Even if you filled an average house completely with water to the top, that still would not be enough to exceed that value.

Calculated, however, no condensation occurs there... and with the perimeter insulation you encompass the entire walls down to the floor – the thermal bridge is very marginal due to the internal insulation and the external perimeter insulation and can certainly be neglected regarding condensation.

Not to be forgotten (but often done) is the other type of equipotential bonding with a fully wrapped floor slab

However, no condensate occurs there according to the calculation...

How must the structure be designed so that no condensate can occur? As far as I understand, it mainly depends on the temperature profile. And at the transition to the base slab, you have the lowest temperature and thus the highest relative humidity.

The modified equipotential bonding is also required with foam glass, right?