Apple watch

Unpair your Apple Watch via the Watch app on your iPhone.

Then Re-pair your Apple Watch,this frees up a surprisingly large amount of storage space.And without any loss of your data.

Or this;

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

Try resetting the Watch. Press and hold the Digital Crown and power button together until you see the Apple logo and then release. The Watch will reboot. If that does not do it, then you can unpair the Watch and then re-pair it again.

Turn the syncing of photos and music off from the Watch app on your iPhone.

No, you will not. The reset I indicated first will not affect your content. And, unpairing the Watch automatically creates a backup and you will restore it when you re-pair the Watch.