With a heavy shield, the cleric must drop/sheathe the weapon in hand in order to cast a spell.
With a light shield, the cleric can hold the weapon in the shield-hand, while casting the spell.
Why not buy a lot of scrolls? At least buy 1 or two each of,
Remove Fear (25gp)
Augury (175gp)
Remove Paralysis (150gp)
Lessor Restoration (150gp)
Dispel Magic (375 gp, or 450 gp for CL 6)
Invisibility Purge (375gp)
Remove Blindness/Deafness (375gp)
Remove Curse (375gp)
Remove Disease (375gp)
Those are the spells you don't usually prepare (or there is not that much merit on casting them by yourself). But those scrolls may save your party when the corresponding situation happens.
Now use all of your spell slots to prepare offensive spells (and/or some buff spells).
Gonna update info at the top, but I just found out the Cleric will be a Neutral Cleric of We Jas. So it will have Rebuke/Inflict instead of Turn/Cure... Not sure if that matters...