How to sync photos+videos to both iCloud and Google Photos while simultaneously deleting from iOS devices?
The following worked for me. For my current purposes, 'iOS device' = iPhone8 running iOS 11.4.1.

Procedure:


Confirm iOS device has uploaded all photos+videos to iCloud Photo Library. (How does one explicitly confirm this?)
Deactivate iCloud Photo Library
Execute Google Photo's "free up space on my device" to remove photos+videos from iOS device.
Permanently delete photos from Photos.app on iOS device; excerpt:

Delete photos permanently

If you want to permanently delete your photos or videos within 30 days, you can remove them from the Recently Deleted album. When you delete a photo from this album, you can't get it back. (An aside: this procedure can be quite cumbersome for thousands or more of photos+videos.)


Open the Photos app and go to the Albums tab.
Open the Recently Deleted album and tap Select.
Tap each photo or video that you want to permanently delete.
Tap Delete.

Reactivate iCloud Photo Library.
Future-captured photos and videos are now both synced to iCloud and Google Photos, while perviously-captured-but-now-deleted-from-iOS-device photos (prior to #2) are all retained in iCloud and Google Photos (?).