No idea what resolutions you are trying to play at so you might try reducing that to get some extra frames.
An RX 480 is starting to show it's age and it looks like by comparison your other parts are a bit slower than that.
Unless you used to get better frames and say, after a driver update it got worse? Then my best guess is you should likely start looking into getting faster components to help.
The fact that when a lot more going on in LOL for instance you get frame drops can be both CPU and GPU performance. Use the performance metrics and see what kind of load the CPU and GPU are under when this happens.
GTAV really likes more than 8gb of ram and suffers with 8gb.