The situation
The house could only be booked through the large listing sites. They set the terms, hold the guest relationship and take a share of every stay.
The owners wanted a place of their own to take bookings, without a second calendar to keep in their heads.
What was built
A booking site of their own. Guests choose dates against one calendar that also carries the listing-site bookings, so a night cannot be sold twice.
A request places a hold. The owners accept it from their phone, the guest pays, and the dates confirm. Holds that nobody acts on lapse by themselves.
The owners receive a text only when something needs them. Every text is built from a fixed template, so nothing a guest types ever reaches a phone.
An owner page, signed into by an emailed link, where they see requests and copy the calendar link for the listing sites.
How it ran
Nine versions, each at a private address the owners could open on their phones. Questions that needed a decision were written as plain questions with answers to tap, and each answer was applied in the next version.