Of course there are solutions. If they have Sonos in other areas, but want it to be even easier, then you need to put an RTI (what you asked about in the first place) in wall touch screen in and create an interface that gives them what they want.
Don't use an Ipad, no reason to give them something that someone could potentially get off the app and require help. Using RTI screen means no way to mess it up, totally custom as you mention above.
Find out what they like and don't like about the Sonos app, and confirm all the possible uses for this room and then use Yamaha or Sonos which ever you feel will work for them. You can even create a nice welcome screen that has the Spa's logo or image which they will like. Might even lead to doing this for the rest of the rooms.