Quick answer
Ask for the meal choice and dietary notes inside the RSVP, per guest rather than per household, with free text for allergies. A second form sent later gets a fraction of the responses and doubles your admin.
Why the second form fails
A follow-up asking for meal choices reaches people who have already done the thing they consider done. Response rates drop, you chase individually, and the answers arrive across email, text and conversation — which is exactly the fragmentation an RSVP system exists to prevent.
Per guest, not per household
Two people at one address routinely want different things, and one of them may have an allergy the other does not. If your form collects one answer per invitation, you will be reconstructing the detail by text message in the final fortnight. Plus-ones need their own answers too, which means capturing the plus-one's name at RSVP time.
Give real options, in the guest's language
List the actual dishes, briefly. "Chicken", "sea bass" and "mushroom risotto (v)" tell a guest what they are choosing; "Option A" does not. Mark the vegetarian and vegan options rather than leaving people to ask.
Allergies belong in free text
A dropdown cannot express "coeliac, and severe nut allergy — cross-contamination matters". A short free-text field can, and that sentence is what your caterer actually needs. Ask for severity where it is relevant: "prefers no dairy" and "anaphylactic to shellfish" require very different kitchen handling.
Only ask when there is something to choose
If the meal is a buffet or a set menu, do not ask for a choice — ask only for dietary needs. Every unnecessary question costs you replies, and a meal-choice field with one option is worse than no field at all.
Handing it over
What a caterer wants is counts by option, plus a list of the individual dietary cases with names attached so the service team can match plates to seats. That is a specific export, not a screenshot — and it should not include your guests' email addresses and phone numbers, which the kitchen has no reason to hold.
Late changes
They will happen. Record them against the guest rather than in a note to yourself, and re-export after the change. One reproducible source of truth is worth more than a carefully annotated document that only you can interpret.