A dropdown selector for sorting layer (name or id), with an "Edit Sorting Layers..." option to directly open "Sorting Layers" tab from "Tags & Layers" inspector where you can change sorting layers.

• Allow Multiple: No

using SaintsField;

[SortingLayer] public string _sortingLayerString;
[SortingLayer] public int _sortingLayerInt;

It can work with ShowInInspector

[ShowInInspector, SortingLayer] private string SortingLayerString
{
    get => _sortingLayerString;
    set => _sortingLayerString = value;
}

It can work with ShowInInspector/Button parameters and return value

[ShowInInspector]  // can be used as params
private int CalcParamSortingLayerInt([SortingLayer] int sceneI)
{
    return sceneI;
}

[ShowInInspector]
[SortingLayer]  // can be used as return value
private string CalcParamSortingLayerStr([SortingLayer] string sceneS)
{
    return sceneS;
}

[Button]  // Works on Button too
private (int i, string s) ButtonParam([SortingLayer] int sceneI, [SortingLayer] string sceneS)
{
    return (sceneI, sceneS);
}