You can rebuild the boot but it’s pretty tedious, easiest is to do a clean install. Just don’t forget to put it on a different system before all that and save whatever you may have important there, or not if you don’t care of losing anything.

Fill it with some junk and delete it and fill it again then format it. Or run a multi pass eraser. Or just take the hard drive out and sell it without one.

That game is pretty old and should work without any issues. Drops like that on 1080p and your setup seems not plausible. Maybe it’s an internet thing or hackers around as i would presume you talk about online part of GTA.

If not, then it could be a ram issue, something not quite working properly maybe OC issue or whatever else frequence thing, but that should give you issues in other parts not just that game in particular.

Maybe a storage issue, your HDD or SSD or the installation of the game. Check for inconsistences.

Or simpler, a setting issue. Maybe being a new pc or whatever of a considerable specs you manually push all settings to max and didn’t use the presets. Even then though there are a few settings that are not that visible when changed but visible in performance. I don’t remember exactly their name but i think it has something to do with antialiasing technology. There is that one that’s upping edges to 4k or whatever in several layers and going full makes it that there are so many layers that it’s very taxing on fps.

You can run the dedicated card only no issue. The idea with laptops with two gpu’s is to switch between them automatically, so when browsing and doing office work to use the less power hungry one and when gaming to switch to the dedicated one. It’s also expected that when gaming you power it with the cable and when on the move working and browsing you use the battery. So it’s really an approach to increase battery use. But you can set it the way you want according to how you use it and like it.