Wall thicknesses in timber frame construction - interior walls and exterior walls?

A bit of leeway everywhere is part of self-planning.

For me too, no matter how certain you feel about the construction method, it makes sense to keep the floor plan neutral in this regard. I would never have to redraw the house from scratch conceptually just because a solid construction main contractor might offer a more convincing deal.

many prefabricated house manufacturers do not allow too many changes to their standard house types.

There are probably a few who do not like more than shifting interior walls; and some who attach prohibitive prices to the changes. But in general, a prefabricated house today is made to measure, so you no longer have to think rigidly in five-eighth-meter increments.