I’ve done this, plugged into iTunes, waved a magic wand and all other possible methods. At best it magically frees 10gbs or so. They ran diagnostics and this had to be uploaded through iTunes as well, waiting too long for the results. My phone entered a boot loop when out of memory despite me using less than half of my phone. Lastly, “On my iPhone” has 10gbs that can’t be located anywhere, I deleted all downloads etc. Not a huge deal but it’s a hell of a bug. When I have too much time I’ll have to do a factory reset but I can’t risk it yet. Offloaded many apps, don’t use them too much so not a problem. It must be due to iCloud or just temp files that refuse to leave.