i have the same problem, i have and can run 5 carsmods but as soon as i add one more its too much memory, i have 32gb of ram and have the packfile and heap adjuster as well as a custom config but no luck,
@zitterkopf First rule out the possibility that one of your existing cars is causing the issue (unlikely but possible). Next, if that doesn't fix it, there are four vanilla combinations you can try. There is only one packfile but there are 2 excellent game configs and 2 heap adjusters. Ensure gameconfig is in your mods folder, and the heap and pack are with the GTA5.exe in the game/root/installation folder.
Option 4 is the only one that works for me after Tuner Update. Nothing else works. Ofc I have the packfile now as well.
Interestingly, even though Dilapidated recommends his own heap with his own gameconfig it doesnt work for me.
If none of those work you need to experiment with your own values. I have 32 GB of RAM, Win10 Pro and well over 200 cars and well over 200 peds etc.