FootIK.cs

using UnityEngine;

public class FootIK : MonoBehaviour
{
    [SerializeField] private float offsetY = 0.125f;
    [SerializeField] private float heightFromGround = 0.5f;
    [SerializeField] private float ikSpeed = 20.0f;
    [SerializeField] private float pelvisSpeed = 5.0f;
    [SerializeField] private string leftFootIKCurve = "LeftFootIKCurve";
    [SerializeField] private string rightFootIKCurve = "RightFootIKCurve";


    private Animator animator = null;
    private Transform leftFoot, rightFoot;
    private Vector3 leftFootPos, rightFootPos, lastLeftFootPos, lastRightFootPos;
    private Quaternion leftFootRotation, rightFootRotation;
    private float leftFootIKWeight, rightFootIKWeight;
    private float lastPelvisPosY = Mathf.Infinity;

    private void Awake()
    {
        animator = GetComponent<Animator>();
        leftFoot = animator.GetBoneTransform(HumanBodyBones.LeftFoot);
        rightFoot = animator.GetBoneTransform(HumanBodyBones.RightFoot);
    }

    private void Update(){

    }

    private void UpdateIK()
    {
        RaycastHit leftHit, rightHit;

        Vector3 leftPos = leftFoot.TransformPoint(Vector3.zero);
        Vector3 rightPos = rightFoot.TransformPoint(Vector3.zero);

        if (Physics.Raycast(leftPos + Vector3.up * heightFromGround, Vector3.down, out leftHit, 1))
        {
            // if the raycasthit is HIGHER than the foot pos in the last frame, the new pos needs to be
            // HIGHER NOW without interpolation - the foot should not vanish in a stone when the stone rises!
            // if the raycasthit is LOWER the foot can get down over time
            leftFootPos = leftHit.point;
            if (leftHit.point.y < lastLeftFootPos.y) 
            {
                leftFootPos.y = Mathf.Lerp(lastLeftFootPos.y, leftFootPos.y, Time.deltaTime * ikSpeed);
            }
            var rotation = Quaternion.FromToRotation(transform.up, leftHit.normal) * transform.rotation;
            // getting the y rotation from the animation looks more natural
            leftFootRotation = Quaternion.Euler(rotation.eulerAngles.x, leftFoot.eulerAngles.y, rotation.eulerAngles.z);
        }

        if (Physics.Raycast(rightPos + Vector3.up * heightFromGround, Vector3.down, out rightHit, 1))
        {
            rightFootPos = rightHit.point;
            if (rightHit.point.y < lastRightFootPos.y)
            {
                rightFootPos.y = Mathf.Lerp(lastRightFootPos.y, rightFootPos.y, Time.deltaTime * ikSpeed);
            }
            var rotation = Quaternion.FromToRotation(transform.up, rightHit.normal) * transform.rotation;
            rightFootRotation = Quaternion.Euler(rotation.eulerAngles.x, leftFoot.eulerAngles.y, rotation.eulerAngles.z);
        }

        lastLeftFootPos = leftFootPos;
        lastRightFootPos = rightFootPos;
    }

    private void AdjustFeetPositionAndRotation()
    {
        leftFootIKWeight = animator.GetFloat(leftFootIKCurve);
        rightFootIKWeight = animator.GetFloat(rightFootIKCurve);

        animator.SetIKPositionWeight(AvatarIKGoal.LeftFoot, leftFootIKWeight);
        animator.SetIKPositionWeight(AvatarIKGoal.RightFoot, rightFootIKWeight);

        animator.SetIKPosition(AvatarIKGoal.LeftFoot, leftFootPos + new Vector3(0, offsetY, 0));
        animator.SetIKPosition(AvatarIKGoal.RightFoot, rightFootPos + new Vector3(0, offsetY, 0));

        animator.SetIKRotationWeight(AvatarIKGoal.LeftFoot, leftFootIKWeight);
        animator.SetIKRotationWeight(AvatarIKGoal.RightFoot, leftFootIKWeight);

        animator.SetIKRotation(AvatarIKGoal.LeftFoot, leftFootRotation);
        animator.SetIKRotation(AvatarIKGoal.RightFoot, rightFootRotation);
    }

    private void AdjustPelvisHeight()
    {
        if (lastPelvisPosY == Mathf.Infinity) 
        {
            lastPelvisPosY = animator.bodyPosition.y;
        }

        float leftOffsetPos = leftFootPos.y - transform.position.y;
        float rightOffsetPos = rightFootPos.y - transform.position.y;

        float totalOffset = (leftOffsetPos < rightOffsetPos) ? leftOffsetPos : rightOffsetPos;

        Vector3 newPelvisPos = animator.bodyPosition + Vector3.up * totalOffset;
        newPelvisPos.y = Mathf.Lerp(lastPelvisPosY, newPelvisPos.y, pelvisSpeed * Time.deltaTime);
        animator.bodyPosition = newPelvisPos;
        lastPelvisPosY = animator.bodyPosition.y;
    }

    private void OnAnimatorIK(int layerIndex)
    {
        UpdateIK();
        AdjustFeetPositionAndRotation();
        AdjustPelvisHeight();
    }
}

I've used three YT videos as reference and inspiration:

https://www.youtube.com/watch?v=rGB1ipH6DrM
https://www.youtube.com/watch?v=MonxKdgxi2w
https://www.youtube.com/watch?v=EggUxC5_lGE

Thank you!