I think I misunderstand how the eldritch invocations that grant spells "using a warlock spell slot".
Sculptor of Flesh Prerequisite: 7th level
You can cast Polymorph once using a Warlock spell slot. You can’t do so again until you finish a Long Rest.
Looking at the Warlock spell list, you are also able to take Polymorph as a spell at 7th level. If I took Polymorph as a spell, couldn't I cast it using pact slots every short rest? What is the point of using this as one of my eldritch invocations?