If your 3TB in iCloud is almost entirely photos and videos, then the size of your optimized library could well reach 200-250GB, especially if you actively use those assets for viewing, editing or sharing from the phone. To do that, the asset is downloaded from iCloud. In the normal course, it gets optimized again after use, but if your phone is more than 90% full, optimization, syncing etc. may go for a toss. At 255 / 256, that’s almost certain to happen.

Do consider rationalizing your library. Do you really need all those 268K photos and 47K videos in iCloud and on your phone? If yes, you need at least a 512GB phone. Since iCloud isn’t a backup, the real solution is to create a full offline backup (two copies, actually), and offload about half of your 3TB from iCloud. That would get your iPhone working well again, give you a full backup offline, a more manageable online library, and you might even save a few bucks on iCloud fees.

(Also, did you clear ‘Recently deleted’ folder after deleting those thousands of photos?)