Rabbenu Bahhya writes in Hovot HaLevavot (Sha'ar HaBitahhon, Chapter 4) that hishtadlut is indeed necessary.
You can see this also by the Jews in the desert. They had to do hishtadlut (efforts) every day to get their manna in varying degrees, but they knew all along that the amount they would get was preordained.
Sha'ar HaBitahhon does not say, however, that the amount of hishtadlut depends on one's level of trust. He writes in Chapter 3 that the system of work is necessary in order to provide opportunities for testing a man in his free will, but if a person passes the tests consistently on all fronts, the hishtadlut he will need to do will diminish until he will not need to any:
"If a man strengthens himself in the service of G-d, resolves to fear Him, trusts in Him for his religious and secular matters, steers away from reprehensible things (such as anger or arrogance - Pas Lechem), strives for the good midot (character traits), does not rebel in prosperity nor turn towards leisure, is not enticed by the evil inclination, nor seduced by the witchery of this world - the burden of exerting himself in the means to a livelihood will be removed from him, since the two reasons mentioned above no longer apply to him, namely, to test him on his choice and to protect him from rebelling during prosperity. His livelihood will come to him without strain or toil, according to his needs, as written "G-d will not bring hunger to the righteous" (Mishlei 10:3)."
Perhaps you can resolve the two by saying that if a person is on a sufficiently high level of trust, it will be easy for him to fulfill the above, since as the Shaar Bitachon says "trust in God is more necessary than all other things for one who serves G-d"