I'm confused, I thought Galaxy S5 phones (whichever variation) came with either 16GB or 32GB of storage. Well at least the ones made by Samsung, is this one a fake knock-off S5?
Well, whatever the case once you take into account the operating system (a must have), if your phone does have only 8GB of internal storage that's simply just too low an amount to begin with. You either have to accept that you need to always be taking active steps to temporarily clear up storage space for daily phone usage, or just lower your expectations accordingly that this phone has limitations.
Maybe it's just time to upgrade to a newer phone? You don't necessarily need to get a flagship model if budget is a factor, there are a lot of good, usable phones now that aren't in that $800+ range.
https://thewirecutter.com/reviews/best-budget-android-phone/