Long story short: rather than mess with manually deleting pics from your phone - even while iCloud Photo Library is disabled! - it's probably easier/safer/faster to just restore your phone. Also, good job, Apple!
Looks like I’m heading down the same route too. Did your health and activity data just synced once you logged into iCloud? That’s the main thing that’s holding me back. And you are right about the iMessages too but like you, I’ve got copies of the messages on my other devices.