Every trade has a list of things it needs from a client and ends up asking for three times over text. A questionnaire asks once, in writing, and the answers stay attached to the event where you can find them on the day.
At least one questionnaire is saved against your account.
Low completion is nearly always length. Ten questions get answered; forty get abandoned. Split the day-of detail into a second one you send closer to the date.
DJs. First dance, entrance, do-not-play, and who is making announcements. Do-not-play is the one that saves the night.
Photographers & videographers. Family group shots by name, plus who is responsible for gathering them. That list is the single biggest cause of overrun.
Coordinators. Vendor contacts and arrival times. You are the one everybody calls when a supplier is late, so having the numbers already is the job.