Is there any way to apply drag to a 3d rigidbody in only 1 direction?

This doesn't seem like it should be too hard to do overall; you just have to do it yourself.

^{For reference, the physics/drag implementation reference is based around a post I made on the subject back in 2016. Nothing has changed since then, to my knowledge, but I'm offering this disclaimer just in case.}
Ensure that your vehicle has *NO* physics drag. Then, in your vehicle script, just include the way you want drag to be applied instead:

 // Example of transferring Rigidbody.drag to a custom variation
 Rigidbody rb;
 float vehicleDrag;
 
 void Start()
 {
     rb = GetComponent<Rigidbody>();
     vehicleDrag = rb.drag;
     rb.drag = 0f;
 }
 
 void FixedUpdate()
 {
     // Example based on PhysX's kinda-lousy implementation
     // for consistent behavior
     rigidbodyDrag = Mathf.Clamp01(1.0f - (vehicleDrag * Time.fixedDeltaTime));
     rb.velocity -= rb.velocity * vehicleDrag * (transform.up + transform.right) * 0.5f;
 }

In this example, the drag is scaled by the tangential (up/right) vectors of the vehicle to avoid dampening velocity along the remaining forward vector.

Thanks! implementing my own drag system is a good idea. while i do not think i will use your exact code it is a good start. sorry for the late response. have a good day!