@DUB95 It can/will be a tedious task for you to do, but I highly recommend you to take this suggestion in consideration and apply it as soon as possible to your modding sessions. It's a pain in the *** when you start doing it and you'll spend a lot of time on it, but is worth it when you've finished because you'll avoid a lot of problems and everything will be more structured. Is the method that I used 2 years ago and is working fine since then
1.- Do a backup of your actual modded game copy (the whole mods folder or the whole game folder).
2.- Take a little modding break and think about how much Add-On vehicles do you have installed, which ones you do normally use and which ones you don't use at all.
2.- Make a list of the vehicles that you don't use and the ones that you won't use ever.
3.- Remove all the vehicles of the list from your PC (.rar, .zip and/or .7zip files).
4.- Create a new Add-On vehicles pack but from scratch, without any vehicle mod installed on it and just the necessary files to start installing vehicles on it.
5.- Install first the vehicles that you like the most and make sure that they all work correctly. As example, install 3 vehicles and try them in-game. If all of them work and the game does not crash, close the game and continue installing vehicles to the new Add-On pack.
6.- Once you finish installing all the vehicles that you want to keep in your game and all of them work correctly, you've finished!
This way you will have all your vehicles in a single Add-On pack, and you won't have to worry about the game DLC's limitation as you do right now. BUT, the Add-On pack shouldn't reach a higher size than 3.5GB. If it reachs that size, make sure to defragment it with OpenIV. If the size does not go down, then you'll have to create an additional Add-On pack and continue installing newer vehicle mods there.