Thank you for the reply!
Oh, I didn't know that I could apply a thermal condition directly in Static Structural, but now I saw this option indeed. Is there any difference in applying Temperature loads in Static Structural instead of in Steady State Thermal?
And I am so embarrassed now, because I didn't have enter the expansion for the Silicon material, for some reason I thought that Workbench had this information already. I added a tabular data from literature for a quite large range of temperatures now, and the simulation started to behave as I was expecting. I guess now it's a matter of refinement. I am so sorry to bother you guys with such a rookie mistake!
Also, regarding the design, the flexures that are supposed to handle the thermal expansion are those in the Aluminum frame. They won't allow free expansion of the Silicon body, but they are oriented in a way that should keep one specific point in one of the faces in position while the rest of the body deforms. The hinged pads (cylindrical parts) are actually to compensate any machining errors between the parts (and to a certain extent it emulates a spherical joint when combined with the flexures in the frame). But if you have suggestions you would like to make, please feel free to do it.
Thank you again for the help!