I have a deity that has a poor favored weapon for a Warpriest, could I take Mauler dedication and at level 7 would I be able to gain expert proficiency in say a Maul or Greatsword for example?
Theres nothing stopping you
Arguibly warpriest is not very good in fighting (and you won't get any warpriest boni on your great-weapon) but this is mechanically perfectly possible and depending on how you play probably even valid (hope you got high str though)
I have a deity that has a poor favored weapon for a Warpriest, could I take Mauler dedication and at level 7 would I be able to gain expert proficiency in say a Maul or Greatsword for example?