In addition to that.. there also another theory..
The carcols file also plays an important role..
When you use addon cars 80% of the time they use the same modkit id in the carcols as other addon cars or even the original cars... car that spawn in traffic sometimes uses randomized customizations and if the addon car spawns with the overwritten modkit id then obviously the game will crash because that car will spawn with incompatible mod parts..
Best fix for this takes time but it works ive done it for my 1200 addon cars..
Open each individual carcols.meta and carvariation.meta and use a free modkit id, thats the first set of numbers in the modkit name eg.
You can search online for free modkit ids for gta v and theres a blog that list them all
Secondly you could just change the modkit name all together to make the game use the default modkit with rims and engine customization