In the SP game，if your character purchase a weapon then you can have it forever，no matter what you do they will always be there. 

But if you want to have online weapons in offline game，then you need a trainer to spawn them.  once you have your weapon，save them in the trainer，next time you start your game，load them from the trainer again to get them back.

To use addon mods，you need to install:  packfile limit adjuster，heap limit adjuster. 

gameconfig is also important but you need to choose right one.  I have 200 vehicles and I am using 3X traffic  3X peds.  which means this gameconfig can handle this amount of addon vehicles.  try to find the one you can use based on how many addons you have.

  This can be caused by using wrong gameconfig or the vehicle mod have some problems. 

   I have 200 cars installed and I am using 3X traffic  3X PEDs.  But if I use the other config like 2X traffic 2X peds then my game can crash after loading.  see if you have the right gameconfig.

If you install your car mods correctly and they are addons， find and install these plugins:

 packfile limit adjuster
 heap limit adjuster
 gameconfig

read the instructions and install them all，then see the problem is solved.

Usually，if you want to play DLC vehicles， the best way to do is spawn them manually by trainer.  If you want them automatically spawn on the street，it is ok but make sure related mod are the latest version and you installed everything correctly. 
For example，I had IVPack installed before，they can spawn IV vehicles and some DLC vehicles on the street. But this mod is currently outdated，so if you install them and the vehicles spawn on the street crashed my game.

Now I can only spawn them with trainer and disable spawn on the street and wait until a new update is available.

   I  had about 200 addons before，including addon vehicles and 15 interiors.  but when I enable mp maps my game crash.  finally I found  some of the interiors cannot compatible with my map，even they are addons. 
 I am not sure if your tree mods are addon? or maybe you enable the mp map at the same time? check them.  If it is then try to stop the tree mod and check your game again:)

You can use a trainer to spawn what you need. I am using simple native trainer now. it can spawn all online content，like maps vehicles and weapons. highly recommended!

 For the whole previous year I have been complaining about my game crash problems. I installed the IVPack and it make my game crash a lot. but now I fixed it. I want to share my experience to help people who still have problems. 

1. IVPack have over a hundred vehicles. which means you will need a new gameconfig for it. I highly recommend you to use the gameconfig made by F7Y0. choose the game config you want, and install it AFTER you install the IVPack. new game config will also change the number of vehicles that spawn on the road. choose the one you like.
2. sometimes even you install the new gameconfig, your game will still crash. usually it will crash when you play the game for about 20-30 minutes. for this problem, you can try to use the vehiclemodelsets.meta. find it at:
   update>update.rpf>common>data>ai
   replace the same file in the mod folder. I don't know how it works. it is a suggestion from F7Y0 but it works well.

I hope this can help people who need help.

If you check my topics, you will find a lot of topics about game crashes. sorry if I put so many similar problems, but this is really weird.
I installed a lot of mods before, and what I see was "unrecoverable fault" or " GTAV exited unexpectly" so I decided to decrease my mods. now I only have IVPack, gameconfig and trainer and replace two weapons. all required files are installed and their version is always the latest.
Now what happens is, my game crashes randomly, like before, but this time there is no messages like "game exited unexpectly" nothing, just crash. I tried many things, like remove the unnecessary programs. turn off the firewall. but still crashes.
I also asked the author of the mods, and they said their mod will not affect the game. the game was purchased from steam instead of other places like Epic.
wondering if there is any player who have the same problem like me?