Restore iPad from iPhone

To restore a device from a backup, the device you want to restore to has to have an OS version equal to or later than the device that made the backup.

You can try when the iPad gets update to iPadOD 16.0.2 or later. You can try when the iPad is first updated to iPadOS 16 which should be available in a few days.

First, I don't think you can restore an iPad from an iOS backup. At least, with these being two different operating systems (albeit, maybe in name only), I wouldn't imagine it being doable. Of course, if someone has been doing this, they will chime in and correct me. No issues there.

Second, and staying with like operating systems, if you try restore from a backup of a newer version of iPadOS to one with an older version, you should see a warning that is going to tell you that you will need to update the older iPadOS to the same version as the backup's.

Ref: Restore an iPhone, iPad, or iPod touch that needs a newer version of iOS or iPadOS - Apple Support