iPhone storage v iCloud storage

iCloud is a syncing service, not an archiving service. iCloud and your iPhone will ALWAYS have exactly the same content so it can be synced to other devices sharing iCloud and can be used to restore a phone that has been erased. Thus, you cannot “back up” messages to iCloud, you can only sync them, and deleting them either will deleted them from other devices.

Optimize will reduce the size of the photos on your phone to about 1/4 of their original file size, but it’s a background process, so leave the phone plugged it to allow it to run. And make sure that Photos is enabled in Settings/General/Background App Refresh.