I don't think that I understood all that you were doing. But, before anything else, we should check if there is a misunderstanding about iCloud. iCloud Photos is a Synchronization Service that keeps all the pictures on the connected devices exactly the same. It's not a transfer service or a backup service. Here's the thing-- since it synchronizes the iPhone Photos Library and the iCloud Photos Library
If you delete pictures on the iPhone, then they will be deleted on iCloud, pretty much immediately.
I can't tell from what you wrote if you understood that, so be sure to check iCloud Photos to see if your pictures are still there. Deleted pictures are kept in the Recently Deleted folder for 30 days, in case you change your mind. If they are selected from Recently Deleted, then they are gone.
I'm not sure about your meaning in "changing from phone to managed photos to manual" Are you saying that you have disconnected the iPhone from iCloud? If that's done before deleting the pictures, then they should still be at iCloud Photos.
In terms of memory measurements, it's not true that the phone or the computer constantly keeps calculating the storage in all its places all the time. I recently erased 150 GB from my Mac, emptied the trash, and re-booted, and the total actually went up a little. The extra room didn't show in the count for at least a day.
One way that people save storage on an iPhone is to use iCloud to synchronize Photos with iCloud, and then choose Optimize Storage in the Photos settings. This may bring the storage of pictures down to less that 20% (maybe even 10%) of what it had been. With this option full sized pictures are kept on iCloud Photos, but the phone maintains only smaller images until editing or printing requires it to download the full size.
