If I may offer a suggestion? Could Markus not simply be the bastard son of one of the kings of one of the many minor kingdoms that arose from the shattered remains of Thule?

This particular king might even have claimed to be the true descendant from the last great king of Thule or rumors and beliefs that suggest the same could be widely held by the common folk.

If you then change the fall of Thule in his backstory to the fall of this minor kingdom (occurring at some when it works best), you'd leave Markus with the same motivation to restore Thule to greatness and a fairly similar claim to the throne.