Hello! After getting some help from a fellow server owner regarding these issues, I wanted to help the community by sharing what I learned. This tutorial will hopefully help you solve the issues mentioned in the title.
Credit to @AntwanR942 for being the one provide me with the help needed to make this post in the first place!
There are three types of situations I will be addressing today:
- Crashes upon entering a vehicle
- Vehicle textures being glitched/ appearing all black
- Physics validation errors
Crashing Upon Entering-
- Crashing when you hit F to enter a vehicle, or in some cases when standing near a vehicle’s seats
- This occurs when a certain vehiclelayouts.meta is incompatible with FiveM servers
Solution:
- Delete the vehiclelayouts.meta (if it is being streamed in your vehicle file)
- Go into the vehicles.meta for your vehicle and edit this line:
You need to replace this with a layout from a vanilla vehicle. You may find the possible layouts under this link:
(Be mindful! Not every individual vehicle has its own layout! Choose whichever layout is closest to your own vehicle! You may have to guess and check a bit.)
Vehicle Textures Are Gitched-
- Vehicle textures appear black or have repeating textures in various locations where they shouldn’t be
- This occurs when the vehicle’s .YTD file is larger than 16 megabytes (FiveM servers have a limit on streaming .YTDs larger than 16 MB)
Solution:
- Open the .YTD file and “downsize” the textures, maintaining their ratio but making the textures smaller, until your file is less than 16 MB
- For example, if you have a texture file in your .YTD that is 4096x4096, you can reduce it in any 1:1 ratio, whether you bring it down to 2048x2048 or 512x512 pixels
(Obviously, reducing the size of these textures will make them look more fuzzy when applied to a vehicle, so choose what you reduce wisely!)
Physics Validation Error-
- When you try to spawn in a vehicle, the game crashes, and gives an error message like the following
- This, as far as I know, this is a model that simply has too many polygons for FiveM to handle
- I have been told that this is an issue that cannot be solved unless the vehicle is re-exported by its creator
Feel free to ask questions below! I’ll try my best to help you solve your vehicle problems 
Big thanks for this guid!