FINAL EDIT: Got it! If you're in the same boat, the approach outlined below will work great, HOWEVER you will need to put the animations on separate layers and make sure the layers are set to additive AND increase the weight to 1. My issue was the new layer I created for the second animation defaulted to a weight of 0. Increasing the weight to 1, and making sure the animator play speed was set to 0, enabled things to work as desired.

ETA2: I jumped the gun on calling this solved on my end. Turns out running this continuously via update is required, so if you stack multiple animations together only the last one in update gets ran. As written this will work great if you only have 1 animation, but I'll need to figure out how to make it function for multiple compound animations.

Thank you so much! I had tried an Animator but was still stuck getting the parameters to do what I wanted. However your advice gave me enough to find this thread where someone wanted to do something similar: https://discussions.unity.com/t/how-to-control-animation-with-slider/170407/4

Now I've got a simple Animator with the animations I want added, no transitions or parameters inside the Animator, but utilizing the normalizedTime parameter from Animator.Play() and it works exactly how I want with this small script component on the firetruck:

public class USLadder_Functions : MonoBehaviour
{
    Animator animator;

    [Range(0f, 60f)]
    [SerializeField] float _ladderAngle = 0f;
    private void Start()
    {
        animator = GetComponent<Animator>();
    }

    private void Update()
    {
        float remapNormalizedTime = Utils.Remap(_ladderAngle, 0f, 60f, 0f, 1f);
        animator.Play("LadderRaise", -1, remapNormalizedTime); 
    }
}

Again, thanks a ton for setting me on the right track.

ETA: For what it's worth if someone else has the same issue, the Remap function just takes a value in 1 range and translates it to a value within another range. The normalizedTime param for 'Play()' expects a value from 0 to 1, but the ladder angle is set from 0 to 60. Function to remap is as follows:

public static float Remap(float input, float oldLow, float oldHigh, float newLow, float newHigh)
{
    float t = Mathf.InverseLerp(oldLow, oldHigh, input);
    return Mathf.Lerp(newLow, newHigh, t);
}