Apple Watch Storage

To Free-Up Storage

For songs, see the "Remove music from Apple Watch" section in:

Add music to Apple Watch - Apple Support

and for pictures, see the "Limit photo storage on Apple Watch section in:

Choose a photo album and manage storage on Apple Watch - Apple Support

Also, try unpairing and pairing your watch again and restoring from the backup just created in the unpairing process. This deletes all temp and corrupted files and re-indexes the file system. This should result in no loss of data and in some cases can free a lot of storage.

Unpair and erase your Apple Watch - Apple Support

Set up your Apple Watch - Apple Support

Also see:

If you don’t have enough space to update your Apple Watch - Apple Support

You need to go into the Watch app on your iPhone and start removing stuff from your watch.

Like any unused watch faces, stored music, images/photos, third party apps you hardly use.