Timber and render

It is OK to render onto eps and it won't crack, provided it is done properly. Obviously the timber structure needs to be rigid. Ply or osb inside and outside, doubled up door pillars etc. Eps is correctly fitted and correct render is used and applied according to manufacturer's instructions. I prefer to use the EWI system.

structure to be solid and locked profesionally(I would let it settle for months-timber to dry,before rendering),also fitting breather membrane to avoid condesation being traped,mecanicahal fixing on the joints and middle line,(also prefer foam on perimeter),then scratch coat plus mesh and top coat,also thickness to be around 20mm to form a solid surface(highly important the render to applied within temperature parameters and procedure,not exposed to sun,etc)

Regarding the render. I would only use k-rend as it’s silicone based rather then sand and cement. My opinion only . Silicone based will move with most cracks.