Hi there, you should turn off your automatic downloads and then try to offload it again, that might work
settings > App Store >turn off "App Updates" and "automatic Download"
and now for your system data, its cache files that your device needs to function properly though and its automically managed by the OS, you can try to reduce it a little bit but it would surely build right back over time, if you wanna clear it all for the meantime, you would need to reset all settings on your device (in settings )if you dont want to there are other things you could do, ie clearing up safari cache properly, deleting large media in Imessage, going into apps itself to clear their cached data. As a last resort, you might want to back up your data to itunes and perform a factory reset ( it would free up alot of system data)
#iphone se refuse to uninstall some apps
sonic nymph