change my screentime passcode

Transfer the photos to your computer first.

Data synced to iCloud or other cloud services should repopulate.

What you're going to lose is in app data. And of course, you'll have to re-download your apps manually.

It's a good time to take a look at what apps you actually use and need on your phone.

You will have to completely erase the device and set it up as new.

Do NOT attempt to restore a backup made after you set the passcode. If you do, you'll end up right back where you started.