As a 15P user if iOS 18.1 drops a public beta is that what the new phones will ship with and skip 18? I don’t want to deal with a version mismatch when the new phone arrives.
New devices ship with the major version for that year, which is a composite of the last Beta RC and some confidential features for the new device (Dynamic Island features for 14 Pro were kept to their own branch, for instance), but ultimately, as the OS is being installed on these devices shortly, pre-release versions, especially pre-Public Beta versions are unsuitable. There have even been issues, including fairly recently, with devices launching and requiring a day-one patch in order to function, especially restore from backup, properly. Issues like that can illustrate how the "iOS [x].0" that the devices come with can vary ever-so-slightly from the version released to the public around the same time. That is to say, any bug fixes made late this month or next month, especially any new features still in Developer testing, won't be present on the launch units. You'll likely be waiting 4-6 weeks for the official release, but simply doing Set Up as New and signing in to your Apple ID will enable you to update to whatever 18.1 Developer Beta is out then, at which point you can use Erase All Content and Settings (but not your eSIM!) and Restore From Backup with your 15 Pro's 18.1 image.