Err... would a 'circular ND Grad' not be a 'Center-Spot' filter?
As said, 'slim' filters are made thinner so that there is less rim protrusion beyond the front element, that can at wide angles cause vignetting... the rim actually blocking the scene you are snapping at the edges of the frame.
Not ALL are 'slimmed' by removing the female thread to accept another filter; though is common, as IF you were worried about wide angle vignetting, you probably wouldn't want to double up too many filters.
But, Polarising filters are 'two-part' and split, a ring attaching them to the lens, then another that allows you to rotate the polarising element to get greatest polarising effect relative to the lens; hence they tend to be 'double thickness' to start with.
Many 'Slim' polarisers then, you are paying for a slimed rotation mount to keep the two-part filter the same width as a standard plain filter.
Here, 'system' filters can 'win' as in a system filter holder, the polarising element can be one-piece, and rotated in the filter holder or the holder rotated on the mounting ring; but filter holders, tend to be far from 'slim' to begin with!
Some suggest buying filters one or two sizes over the largest filter size for the largest lens you have, then buying step-down adpators to whatever each of your lenses takes.... means that you only need one of every filter, rather than one of each filter for each lens; and larger than the largest element, the filter rim, ought to be well out of the way of risking vignetting... provided you only use ONE step-down ring fron whatever size...