The upgrades hangs on install? I tried resetting this iphone to original, tried installing using itunes with no progress as it hangs on the install progress bar. Maybe 15.2 is not compatible with the SE 2016?
Over 6gb should be enough as the file size is about 2.5gb. Actually I tried that, reset the phone, wiped it clean and down to just the basic apps. Even with low space you should be able to download to Mac desktop and install using itunes.