@TheMurderousCricket I didn't remember that it requires Rage Plugin to work. That simplifies the "problem" a lot, because to use LSPD:FR you need to use the Rage Plugin executable file, instead of any of the original game executables (GTA5.exe, Play.exe or GTAVLauncher.exe).

You don't need to duplicate the game folder then, because the Rage Plugin script mods like LSPD:FR load only when you use it through its own EXE.

@TheMurderousCricket LSPD:FR does need of an specific script file to be loaded, right?

If yes, then you'd only need to remove the required script and it won't (or at least shouldn't) load. If it works that way, you just need to do that when you don't want to use LSPD:FR.
If you prefer to keep things individually or the previous method doesn't work, then yes, you'll have to duplicate the game folder.

@TheMurderousCricket You can create 2 "mods" folders and install the mods separately on them. When you want to don't use one of them, rename it to something else than "mods", for example "nomods" and OpenIV won't recognize it. Keep in mind that you'll only be able to use one "mods" folder, not both at the same time.

Of course you'll have to update both folders everytime that the game gets updated.

@AloneArchANGEL I've had more than 400 added vehicles installed without problems. It is not the fact of how many you can put, but how do you install them and which do you use.

I recommend the following:
1.- Don't exceed the 3,5GB/Add-On vehicles folder size. More than that size can corrupt the DLC and make your game crash.
2.- Defragment the DLC folders everytime you make vehicle model changes, like removing or adding a vehicle.
3.- Install vehicles that were tested by other people and worked for their game. If you do that and there's is a problem when you installed it, then the problem is not the mod, but your fault in the installation process.
4.- Do everything carefully, not rushing the process. You must be sure that everything you do is correct and you did not any mistake. Or if you're not sure about an step, keep in mind that if the game creashes maybe there's a problem with such step.

@AryannSharrma We don't support Pirated game installations. They're not compatible with mods and pirate copies are illegal. Buy a real copy before asking for help...

@JoeeHere

Never update the extratitleupdatedata.meta file if is not a Rockstar Games game DLC update. It's useless for Add-Ons, since Add-On mods never will be patched in /update.rpf/dlc_patch/ folder.

For dlclist.xml file, just add a new Add-On line to the original file of your game each time you install an Add-On and never replace it with the one that comes with mods, since they can be outdated!

@Killy1 I suggest you to don't use the GTA Realism mod anymore.

The reason is simple:

The mod is outdated and that's causing your game crashes. GTA Realism mod needs a serious update for the current game version, and it's not gonna be released anytime soon.

@StarFisher001

content.xml is the file that specifies the paths of the files that the game will load. In other words, when you start your game, during the loading screens it goes to every one of the paths that you can find in any content.xml and reads the content of each file that's specified there.

setup2.xml, if I'm not wrong, are the properties of a DLC. The name of the file is already telling us that's an specific "setup" for it. There's not much to do with it, but you can take a look here and see how it's used for an Add-On. Is the best way to understand the things. "Trial, error, trial, error, trial and finally success".

dlc_patch folder in update/update.rpf/ is the place where Rockstar Games places the modifications (patches) they do to a previously released DLC. They do that to avoid having to download the whole DLC content again, so if a DLC is 1.5GB size but the update only takes 200MB, they'll be able to just update the 200MB. The original DLCs are found in update/x64/dlcpacks/, while the updates are located in update/update.rpf/dlc_patch.

I hope this helps you to understand a bit better how works the basic game stuff.