Pathfinder 2e Character Discussion

So I understand that until you get your first magical staff. you make do with the makeshift one you created.

Questions:

• Do you have to have to expend a spell slot in order to cast cantrips from the makeshift staff? The linked rule says you don’t have to expend any charges, which seems to imply that the staff must have a charge in order to cast any cantrips.
• I assume that once you add the cantrip and 1st-level spell to the staff they can never be changed. The text on page 142 of the APG doesn’t say that you can, but I’m new so don’t know if there’s some other text elsewhere that allows this.
• If true, and considering that these spells can never be changed, I’m thinking of Shield and Mage Armor as my spells. However reading through spells True Strike also seems to be a good option. Although not sure how good that would be for my Illusionist specialist (with Enchantment as other spells I’m focusing on). Thoughts?

Thanks in advance for any guidance you may have!