@Siferiax and others have shared fantastic work related to queries for task management in Queries for task management. Consider looking through that discussion for many outstanding examples.
To combine sort-by you can use juxt. Look also into map.
A variation of the following might do what you need:
:result-transform (fn [result] (sort-by
(juxt
(fn [h] (get h :block/priority))
(fn [r] (get-in r [:block/page :block/journal-day]))
)
))
EDIT: for anyone looking at this in the future, note the correction by @Siferiax below. It should be:
:result-transform (fn [result] (sort-by
(juxt
(fn [h] (get h :block/priority))
(fn [r] (get-in r [:block/page :block/journal-day]))
)
result
))