Why can't you backup? iCloud isn't bs, by the way. Either way, you can't selectively go into System Data and remove things. Over time System Data builds up and occasionally accumulates some corrupt data as may be the case on your iPhone. The simplest way to reduce it back to normal levels is to backup your iPhone, then restore the iPhone from backup. If you don't or can't use iCloud, then use your computer with Finder (Mac) or Apple Devices (Windows) and backup your phone there, then restore the phone from the backup.
This link explains how to do that --> Restore your iPhone, iPad, or iPod touch from a backup - Apple Support