It looks like the original structure is already close to the minimum, which might be quite flat. So the algorithm is struggling to find a clear direction to move in to lower the force even further. I would suggest lowering the SCF convergence criterion (generally for all geometry optimizations) since what could be happening is that the forces are just not accurate enough to distinguish clearly so close to the minimum.
You can also consider a different way to optimize. This is a highly symmetric structure, so the atoms should not really move (that is, their fractional coordinates should not change). Thus, the proper constraint is rather to keep the space group (constraints=[SpaceGroupConstraint()],) and optimize the hexagonal lattice constants a and c. (Tip: First symmetrize the structure to move the atoms into their exact positions [[2/3,1/3,0],[0,0,1/2]] instead of the approximate coordinates you have now.)
