Hello eniola83,
Thanks for using Apple Support Communities. It's our understanding that System Data is using a larger amount of storage on your iPhone 12. You've done a great job trying to get this resolved. We're happy to help.
If you haven't already, make sure that your iPhone has the latest version of iOS 15.
Update your iPhone, iPad, or iPod touch
If the issue persists, restart your iPhone. Restarting can resolve many unexpected behaviors.
Restart your iPhone
Let us know the results. Be sure to include specific error messages and any other troubleshooting steps that you may have tried.
Have a great day!