"I have been through that post, but it doesn't seem like there is anything that helps me other than a slim definition of symmetry and circular symmetry. "

>> this is the reason I recommended taking a look at it. to ensure that you know which BC that you should use.

"Are there by the way a fulfilling description of what SW does when applying symmetry (more than is described in SW Help)?"

>>as an educational user you are somewhat limited, you can speak to your instructor they may have access to the solidworks KB that has additional information. but also, this BC uses the standard definitions of symmetry for FEA.

"What I meant regarding the restraints is that if I fix both cutfaces, then the disc won't be able to deflect as seen here:"

>>this is why i recommended that this is unlikely a good BC for your case. remember that symmetry requires that in addition to the geometry and loading conditions to be symmetric, the results/behavior also need to be symmetric.