Help removing "system data" because it taking up all my storage space

The trouble is that "system data" could mean anything. It might even be referring to the total size of the system partition, in which case this would be a total red herring because nothing you do will change the partition size. If it's unchanged immediately after a factory reset I suspect it might be something misleading like that (unless you have stuff automatically restoring from a network backup after doing the reset).

So just to make sure we're clear what your storage situation is, could you install a small app called "Storage Truth" from the Play Store, run it and post the output here? There's a sticky post about this app in this forum. All it does is print the sizes and free space in each of your partitions, but it's usually clearer than system "storage" menus which vary between manufacturers and are often quite confusing.

 

I seem to remember someone "Famous" is the author of that app

* Storage Truth - Android Apps on Google Play

 

Thanks for the link.

Back on topic: As far as I know the /system partition is a fixed size as already noted. To gain any actual user space, I remove the bloat from /system then convert my "must have" user apps to system apps. TiB makes this quite easy, but isn't free. I know of one in the F-droid repo that is. I believe that it's called system/app/mover. Just be careful because once removed not even a factory reset will bring it back. I recommend TiB (unless you have a recent nandroid via custom recovery) so you can backup any apk prior to removal.
In a nutshell, any apk you move from /data/apps to /system/app is now available for other usage.

 

You've got to be rooted to change user apps into system apps, and you've got to really trust the apps you are making into system apps.