...that don't have phone numbers that are redundant with other client profiles. This is a runaway issue. Some fix to this is very much needed!

Not sure what the best solution would be but some type of "parent" or "primary" account status could be granted. These accounts could be linked with other profiles and have freedom to book appointments for them and those accounts would have access to use the primary account's payment types as their own defaults. The caveat would be we would be placing the organizational work on either the clients or the business. Another option would be sub-accounts of the primary accounts for these guests that automagically soft merge profiles that are guest profiles so that not so much redundancy gets created.

Simple logic after a couple instances of group booking could determine, if [primary] books [guest1] [guest2] >twice, merge those [guest1] with [guest1] and [guest2] with [guest2] continuously. But to avoid conflict keep the guest type profiles a separate type that cannot be merged with full client profiles.