Welcome. To combine multiple transformations, should generally nest the first ones inside (as the result input of) the last ones. In your case, something like this:
:result-transform (fn [result]
(sort-by
(juxt
(fn [d] (get d :block/deadline))
(fn [d] (get d :block/priority))
)
(map (fn [m]
(assoc m :block/collapsed? true)
) result)
)
)
Whether this is the result that you want, it is a separate matter. But it does the combination/unification and you can continue from here.