Is it legit to use this disconnect this way
There is one major problem with your plan -- GFCI circuit breakers have LINE and LOAD terminals, and thus cannot be backfed according to Code and their Listings (according to this Square-D document, you'll fry the trip solenoid if you try).  What I would do instead is change the existing 30A breaker in the main panel out for a 30A GFCI and then backfeed a disconnect switch installed in place of the subpanel's GFCI instead of using a backfed breaker or main lugs at the subpanel.  This provides a single disconnecting means for the branch circuits at the subpanel.