The contract
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Four clauses do most of the work. Everything else in a filling agreement is ordinary commercial terms.
Who owns the formula
Say it explicitly: the formula is yours, it is confidential, and it may not be made for anybody else.
Without that, a co-packer who improved your formula during scale-up may reasonably regard the improvement as theirs, and you have a product you cannot take elsewhere.
Ask also whether they will hand back the scaled process, not just the product. That document is what lets you move.
Who is named on the label
Manufacturer, packer or distributor. It is a one-line decision with a real consequence: it says who is responsible, and it is expensive to change once labels are printed.
Settle it before artwork rather than discovering it at proof stage.
Acceptance, and what happens to a bad run
Written acceptance criteria against your specification: fill weight tolerance, appearance, color against a standard, mechanism function.
Then what happens when a run fails them. Who pays for the materials, the components, the labor and the disposal, and who decides.
Without this clause a grained or short-filled run becomes a negotiation at the worst possible moment.
The exit
Notice period, what happens to your materials and components held at their site, and whether they will supply a final run while you move.
Also: who holds the batch records. You need them, they are your evidence about your own product, and a departing relationship is a bad time to ask for them the first time.