Is there a mod that allows NPCs to use cars from recent DLCs like "MP cars in SP" mod did?
Yeah, just it
I suggest Traffic Plus+. It add's online-exclusive cars to traffic, plus any compatible add-on cars that you install.
just modify your popgroups. My popgroup generator makes it a lot easier for you. Significantly more efficient than Traffic+ as the game spawns them ambiently rather than a script forcing the spawn every few seconds