Deleting photos does not always make storage available immediately. The first place to check is the Recently Deleted album, because removed photos and videos can remain there for a period before permanent deletion.
Check Recently Deleted
Open Photos, locate Recently Deleted and review its contents. Only permanently delete items you are sure you no longer need.
Review iPhone Storage
Open Settings → General → iPhone Storage. Wait for the storage graph and app list to finish calculating. Large messaging apps, downloaded media, offline videos and browser data can be more significant than the photo library.
Restart after large changes
After deleting a very large amount of data, a restart can help the storage display refresh. Avoid installing “cleaner” apps that promise access to system areas iOS does not normally allow them to clean.
Keep a backup
Before permanently deleting important photos or files, make sure a separate backup exists and that you can actually access it.