iCloud Photos

If by "getting off" you mean deleting (Ie you already have them all on your mac or ipad) - it might be easiest to re-initialise / reset your phone.

Though you then lose all other data that isn't synced with iCloud.

Or you can try this method (but again I think life will be too short)

https://www.businessinsider.com/how-to-delete-all-photos-from-iphone?r=US&IR=T

Hi Shane

Holy Cow - that is one huge library. I think you are going to need to split it up (as it looks you have already realised).

First thing, you need to stop the iCloud syncing - because since you have turned iCloud off and or on - it will try a complete new sync, and with 4TB you may well not live long enough. 🙂.

The best way I can think of for this is to create a new empty library, designate it the system library, and then close it. (This will stop your other libraries being the system library - and hence stop them trying to sync to iCloud). Close the photos app altogether

You should then be able to turn off iCloud photos in system settings.

Do you have a backup - if not get one sorted now. You are going to be processing huge amounts of data and the chances of things not going missing during that process is close to zero.

Backup sorted? I would then split up the big offline library. You can do this by creating copies (You'll need some big external drives - NOT network drives - and correctly formatted APFS or MacOS extended.)

You can do this by creating copies (Yes, you are in this for a long haul) and then deleting the photos you DON'T want in each copy. The fastest way may be to take one copy (so you now have two) then delete half the photos from each (Eg before/after a particular date). Then copy those two libraries and repeat. So as time goes you are copying smaller and smaller libraries.

Once you've done that to a point where you have manageable archive libraries, you can create a new library for new/working photos, which you can then sync with the phones. Though if you have 4TB worth of photos in the phones (presumably with optimise phone storage set), then there will be some management needed there also.