Deleting synced Aperture photos from iPad (before iCloud Photos migration)

I'm finishing up the long process of migrating from Aperture to Photos.app/iCloud Photos, getting to the iPad part of it, and having trouble.

How can I delete or remove the synced Aperture photos from iPads using current macOS and iOS? My iPads still have synced Aperture photos on them, and I'd like to remove them before enabling iCloud Photos syncing, to avoid duplicates on either the device or in my iCloud Photos library.

The worst-case scenario I want to avoid is that enabling iCloud sync somehow causes the synced Aperture photos to get imported into my iCloud photos and I end up with duplicates. My library is 10,000 photos, and de-duping that would be a huge pain. Or even worse, if it somehow caused deletion of the corresponding iCloud photos.

I migrated to Photos back in 2023. I didn't turn off the "sync photos to your device" option for my iPads in iTunes before doing the Aperture to Photos migration. Then I upgraded macOS on my Macs to a version that didn't support Aperture, and kept iOS current. Now there doesn't seem to be an option to remove the Aperture photos; the "sync photos to your device" checkbox in Music's Sync Settings for my iPads is already unchecked.

Anyone know what to do in this situation?

Background

I have an Intel iMac Pro "angharad" running macOS 15.2, three iPads running iPadOS 18.2, and an iPhone running iOS 18.2. The iPads: iPad mini 6th gen, 11" iPad Pro 3rd gen, 13" iPad Pro 4th gen. My photo collection is in Photos on the iMac, and iCloud Photos sync is enabled on the Mac and my iPhone. 10,064 photos and 14 videos in it, according to the count shown in Photos.app on my Mac and iPhone.

The iPads, in their Photos app, show between 6,900 and 8,452 photos, and they only go up to September 2023, aside from photos and screenshots taken locally on the iPads. That's reasonable, since I selectively synced over only selected albums from Aperture, and didn't get the album selections the same on all devices, and Sep 2023 is when I migrated to Photos. They were synced from my Mac angharad using iTunes back when it was running macOS 10.14. In Photos on the iPads, there is no "delete" button displayed on the old Aperture-synced photos. The iPads are backed up locally to my Mac, and also backed up to iCloud.

I tried connecting my iPads to my Mac today to remove the synced photos by turning off syncing. I connected them using a USB-C cable. In Music on the Mac, in its Sync Settings for the iPads, the "sync" checkboxes for music and maybe movies are checked, but the "sync photos" checkbox is unchecked.

I am reluctant to try turning on "sync photos to your device" and then turning it back off, because I fear that might cause the old photos synced from Aperture to get copied into my Photos or iCloud Photos library. (Maybe that doesn't make sense, because importing photos from an iPad to iPhoto/Aperture used to be done with a separate "import" process in iPhoto, and it probably works similarly now? But still. I just don't understand this current cloud & Photos sync model very well.)

Maybe I should just try wiping the iPad and restoring it from a backup? It looks to me like the synced photos are not included in iPad backups, because my local backups are like 6 to 9 GB, and I've got 14 to 17 GB of photos on them.