So basically there is absolute nothing allowable to improve the graphics of the game if you also want to play online safely.
Cheat modders with their money spawning and god modes have ruined it for everyone by making R* take a blanket ban approach. Yet the online game is still packed full of cheaters because they just buy new accounts every time they get banned anyway.
NVIDIA offer a game filter (nvidia freestyle) as part of Geforce Experience which can force certain effects at driver level. But it has limited options and does not offer any anti aliasing (which imo is the biggest issue with this game). Plus it is quite glitchy and obviously limited to NVIDIA GPUs.