@finsikesi64 Great news for you. Go into your GTA5 installation folder, also called the root folder.
In there you will see a file called dinput8.dll (microsoft direct input).
Temporarily rename this file anything you want. For example rename it with a dot at the beginning, .dinput8.dll (will be easy to find) or dinput8.bak - whatever you want.
Start your online game. That's it. Next time you want to play SP, just rename it back to the correct dinput8.dll name.
Want to do this automatically, create a batch file that renames and then launches GTAV.
What happens with Steam (not sure about Epic) if you forget to do this and accidentally try to go online. Your game will be deleted automatically and you will be banned for life. Ok, just kidding. Nothing will happen. You will simply get a warning that you are trying to go online with a modded game.
So rest easy. No need for duplicate games. One file renamed. That's it. That's all.