The same link becomes the invitation
This is the part that makes the page worth more than the message. Once the venue is booked and the timings are real, you edit the same event: add the schedule, the address and directions, the dress note, the full reply form with meal choices and plus-ones inside whatever limit you set. Publish again, and the link you sent in March now shows the invitation.
Nobody needs a second link, and the chats that carry the old one are not suddenly wrong. When you send the invitation news, it is a nudge to reopen a page some people already have, rather than a correction to something they saved.
Until you publish, nothing half-finished escapes. A draft is not visible to guests, so the save-the-date page stays exactly as short as you intended while you build the rest of it behind the scenes. If the notice is for one group only, password protection keeps it there.