How to customize TinyMCE at runtime https://devblog.gosso.se/?p=824

CustomXhtmlStringEditorDescriptor.cs

using EPiServer.Core;
using EPiServer.Shell.ObjectEditing.EditorDescriptors;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Gosso.Episerver.Business.EditorDescriptors
{
    [EditorDescriptorRegistration(TargetType = typeof(XhtmlString), EditorDescriptorBehavior = EditorDescriptorBehavior.PlaceLast)]
    public class CustomXhtmlStringEditorDescriptor : EditorDescriptor
    {
        public override void ModifyMetadata(EPiServer.Shell.ObjectEditing.ExtendedMetadata metadata,
            IEnumerable<Attribute> attributes)
        {
            if (HttpContext.Current.User.IsInRole("Bloggers")) // do your own logic
            {
                if (metadata.EditorConfiguration["settings"] is Dictionary<string, object> settings)
                {
                    var toolbarList = ((IEnumerable) settings["toolbar"]).Cast<string>().ToList();
                    if (toolbarList.Count >= 1)
                        toolbarList[0] =
                            "bold italic strikethrough styleselect | epi-link image epi-image-editor | customfaktabuttonplugin customimagebuttonplugin | bullist numlist | removeformat | searchreplace fullscreen";
                    if (toolbarList.Count >= 2) toolbarList[1] = "";
                    if (toolbarList.Count >= 3) toolbarList[2] = "";
                    settings["toolbar"] = toolbarList;
                    settings["menubar"] = "false";
                }
            }

        }
    }
}