Higher resolution Sprite Shapes than unity allows in the normal editor. Cranking it too high can lead to tesselation errors. If there are errors, turn it down a little.

HighResSpline.cs

using UnityEngine;
using UnityEngine.U2D;

[DisallowMultipleComponent]
public class HighResSpline : MonoBehaviour
{
    [Range(4,128)]
    public int resolution = 16;

    void Start()
    {
        var controller = GetComponent<SpriteShapeController>();
        controller.splineDetail = resolution;
        controller.colliderDetail = resolution;
    }
}