Ring beam - Insulation for 5cm U-shaped shell

So I have lived for 20 years in various prefabricated buildings and also drilled into exterior walls. I have never come across anything like aerated concrete there. Are you sure?

Purely mathematically, you will do better with Resol boards and 8 cm, but for me, at least regarding the insulation thickness, a sufficiently oversized ring beam would be more important.

Either double-layered with insulation in between. Then no lightweight concrete was used,

I'm not quite sure what you mean. Do you mean I should simply go for 8cm insulation with WLG 035?

Exactly the other way around.
The static load-bearing structure of the house was calculated with the existing ring beam. If you now go from 5 cm insulation to 8 cm, something will have to be removed. Either part of the Poroton bricks (which would only have a minor impact on the U-value of the wall at this point) or the ring beam will be 3 cm narrower. The latter would have a greater energy effect but also affect the statics.

Therefore, if desired, I would rather switch from XPS with WLG 035 to Resol with WLG 022. This would not require any changes to the Poroton bricks and the statics regarding the ring beam would remain unaffected.

U-value = 1/(1/Lambda * d)
XPS 5 cm: 1/(1/0.035 * 0.05m) = 0.7 W/m²K
XPS 8 cm: 1/(1/0.035 * 0.08m) = 0.44 W/m²K

Resol 5 cm: 1/(1/0.22 * 0.05m) = 0.44 W/m²K
Resol 8 cm: 1/(1/0.22 * 0.05m) = 0.28 W/m²K

In addition, there is the thermal resistance of the Poroton bricks, plus the internal and external heat transfer coefficients, so that with the 5 cm Resol variant you end up at about 0.25 - 0.3 as the U-value at this point and with 8 cm you probably get somewhat below that.
However, a difference of 0.05 W/m²K over an area of maybe 10 m² has no noticeable effect on the overall heat demand of the house.