Delete iPad photos without losing iPhone photos

The only way to save storage on your device by using iCloud Photos is to enable the option "Optimize iPad Storage" or "Optimize iPhone Storage" in the Settings on the device where you want to save local storage. Then iCloud will manage the storage and remove the high resolution versions of the photos and videos you have not used in a long time., whenever more free storage is needed. The optimized items will be replaced by a smaller thumbnail for browsing on your device, so you can download them again from iCloud. You cannot control what will get optimized, it is fully automatic.

But even the optimized versions need local storage. If all items need optimizing working with Photos will very slow and no joy, because all items will need downloading from iCloud. So I would not let the iCloud Photos Library get larger than will fit on your device with the least storage, with a moderate optimization. As a rule of thump - letting the library shrink by optimize storage down to 20% will be o.k. , but going down to 10% will start to get annoying. So, for example, if your you can afford 20GB for an optimized Photos Library on your iPad, don't let your iCloud Photos Library grow larger than 100 GB. in iCloud. Your device with the least storage will be the bottleneck.

Some background on iCloud Photos: Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community

You cannot do that as long as they are both connected to the same iCloud Photo Library, since its main purpose is to keep the content of all connected devices the same.

léonie wrote: … letting the library shrink by optimize storage down to 20% will be o.k. , but going down to 10% will start to get annoying. So, for example, if your you can afford 20GB for an optimized Photos Library on your iPad, don't let your iCloud Photos Library grow larger than 100 GB. in iCloud. Your device with the least storage will be the bottleneck.

I think that the 20% thing is a good number. I don't "Optimize" on my iPad, since it has plenty of storage, but I do on my iPhone. But, because I never edit or print pictures from my iPhone, then I'm not bothered by my Photos Library being even as little 5% of my iCloud Library, which it is, apparenlty. If I do go into edit mode on my phone for some reason, I then every time have to wait for a download from iCloud. But as long as I don't edit, I can rapidly scan through pictures and they look great on the screen, even zooming in. I'm not sure how much it matter that my iPhone is only 40% full, with over 80GB free.

I'm not recommending anything; I'm just offering a data point, here…

That you for this added information, Richard, a good example. I think the percentage depends on the size of the individual original image and video files. If most of our media in the library are huge RAW files or videos, the optimized versions will be tiny compared to the original files in iCloud and we can easily get down to 5%. But if the originals are already small, perhaps just HEIC versions with 3MB instead of TIFFs of 80MB, the optimization will not be able to save as much storage as when we are using RAW formats.

I am archiving my RAW files on my Mac and are not keeping them in my iCloud Photos Library. In iCloud I am having mostly the HEIC versions of the edited versions.

léonie wrote: … I am archiving my RAW files on my Mac and are not keeping them in my iCloud Photos Library. In iCloud I am having mostly the HEIC versions of the edited versions.

I have few if any RAW files in the Photos Library-- I keep all the RAW files in Finder folders, eventually moved to external drives. Most are jpgs from my Nikon or jpg Lightroom exports from RAWs, and they have way more pixels that are required by the iPhone's screen. The average file size is about 8MB, apparently, with the videos being mostly less than a minute (though some are a few hundred MB.)

It's interesting that so much optimizing is done when I have 80GB free on the phone.