Welcome to the Bunnings Workshop community @stasiagarraway. It's amazing to have you join us, and many thanks for your question on repairing shower bases.
It sounds like our helpful members will need to see some images of the damage you're trying to fix to offer a solution. A cracked shower pan and spongy floor do not sound great, and I suspect this is something that expanding won't fix on its own. Before repairing the damage, my first concern is the integrity of your waterproofing membrane/shower pan. A spongy floor would indicate significant damage to the underlying structure and could suggest a breached waterproofing membrane/shower pan.
From what you have described, this sounds like an extensive repair is needed. The tiles and shower pan will need to be removed, any damage to the floors structure assessed and repaired, waterproofing membrane re-applied/shower pan replaced, and tiling redone.
I look forward to seeing some images of the damage and offering some suggestions for repairing the area.