Also somatic components to a spell can be used while holding something so no problem there. No regrip action necessary. Free hand is only needed for material component spells or casting a spell through an item.
"Source Core Rulebook pg. 303
A somatic component is a specific hand movement or gesture that generates a magical nexus. The spell gains the manipulate trait and requires you to make gestures. You can use this component while holding something in your hand, but not if you are restrained or otherwise unable to gesture freely. "
But your initial assumption is right.
"Source Core Rulebook pg. 287
This lists how many hands it takes to use the item effectively. Most items that require two hands can be carried in only one hand, but you must spend an Interact action to change your grip in order to use the item. The GM may determine that an item is too big to carry in one hand (or even two hands, for particularly large items)."
To me that indicates dropping the hand would most likely be a free action. Regripping and using that item or weapon again would be an action.
The rules for this are on Table 6-2: Changing Equipment (CRB, p. 273).
Removing a hand from your two-handed weapon is a Release Action which is a free action.
Returning to a two-handed grip is an Interact Action which takes one action.
Yeah, basically, taking your hand off and regripping costs an action, but you rarely need to take your hand off because somatic components can be done with your hands full.
That still leaves the situation of needing hands free if you're doing material or focus component spells. But as a cleric, you're allowed to substitute a focus component for material component spells. So if you find that you're using a lot of material/focus spells, you can pick up the Emblazon Armament feat and be done with the whole problem.