I am sorry that it is difficult to figure out how not to take that IOS update and get the space back. I would think that one of our local Apple experts could shed some light on the subject. Android and Microsoft have similar issues, but I know how to disable these from being downloaded.
I have no experience with Apple products since the very early Macs... I purchased a flatbed scanner which came with Photoshop 2.5 in 1993. At that time digital cameras still had very limited resolution and were very expensive. My best friend purchased a similar package for his Mac. He was quite sure that since Photoshop was originally for Macs only... that his system would be superior to mine for photo-editing. After spending considerable time with each system even he had to admit that my system was far more capable than his. We went through similar iterations when we got into video editing.
I do not like being forced into “rooting” Android devices with the connotation that we are doing something wrong that may void our warranties. The fact that Apple is even more restrictive about letting users control the nuts and bolts of their system is a great turn off for me. It reminds me a lot of strong central government control. The feel the first Macintosh TV ad Why 1984 Won’t Be Like 1984 was one of the misleading commercials of all time. You can watch it on YouTube. Apple has always maintained strict central control of its product lines.