I did try custom OS in the past (Even overclocked my original Galaxy S), but they all had issues. Or were slow on updates and so on. Out of the box works best for me, I already tinker enough at work, I want my phone to just run :)
Not a fan of screen protectors, they all don't have great reviews, even the expensive glass ones. I also use the phone without a case, so the protector would make the handling of it worse unfortunately.
That was released 6 years ago.. so what you have might be a bug that has been long fixed. Or the phone using WiFi for location services (to help GPS).
Either way, you need to update (custom ROM maybe?) or get a new phone. Using Android 8 is a security risk.