Ok, this is going to have paid apps, but I will guarantee you that you will enjoy these apps and will appreciate it. I would change your whole theme to black, and by doing that download Substratum Theme Engine, and download Swift Black. Try learning how to use it, it's very easy once you know. It'll convert mostly everything black, including Instagram, YouTube, and more. I will guarantee you will love it because everything becomes black, so it'll use less pixels.
Afterwards, download Package Disabler Pro. What that does is force disable things you normally can't in settings, and by disable I mean completely disable. Try disabling things you don't use like Bixby for example. You can disable all Bixby stuff, and that will save you a lot, because Bixby is constantly running in the background. In the app just put on search "Bixby", and disable all which are around 5-15 stuff. Reduce resolution size, and use black wallpapers.
Those things will help a lot