This doesn't make a whole lot of sense...
I’ve provided the 90 second animation that was agreed
they keep saying they are working on the content still
What content if you've delivered the agreed upon work? At face value, based upon your post, this sort of sounds like a stalling tactic by the client.
Send an invoice now.
If you've done the agreed upon work, invoice. All they can do is question you about the timing of the invoice. In which case merely explain... you agreed to complete an animation. That animation is complete. Payment is due. If they need further work, you are happy to discuss that matter.
I will often send an invoice with a link to final deliverables. That way clients get both at the same time and can not claim they missed an invoice if they want the final deliverable.