How to make a Rust building grade limits plugin (Oxide)

using System;
using System.Collections.Generic;
using Newtonsoft.Json;
using Oxide.Core;
using UnityEngine;

namespace Oxide.Plugins
{
    [Info("Building Grade Limits", "RustPluginGenerator", "1.0.0")]
    [Description("Blocks building upgrades above a configurable maximum grade, with per-grade permissions.")]
    public class BuildingGradeLimits : RustPlugin
    {
        private const string PermUse = "buildinggradelimits.use";
        private const string PermBypass = "buildinggradelimits.bypass";

        private PluginConfig config;

        private class PluginConfig
        {
            [JsonProperty("Default Max Grade")]
            public string DefaultMaxGrade { get; set; } = "Wood";

            [JsonProperty("Ignore Admins")]
            public bool IgnoreAdmins { get; set; } = false;

            [JsonProperty("Announce Blocked Upgrades")]
            public bool AnnounceBlocked { get; set; } = true;

            [JsonProperty("Show Grade In Message")]
            public bool ShowGradeInMessage { get; set; } = true;

            [JsonProperty("Grade Permissions")]
            public Dictionary<string, string> GradePermissions { get; set; } = new Dictionary<string, string>
            {
                ["buildinggradelimits.wood"] = "Wood",
                ["buildinggradelimits.stone"] = "Stone",
                ["buildinggradelimits.metal"] = "Metal",
                ["buildinggradelimits.top"] = "TopTier"
            };
        }

        protected override void LoadDefaultConfig() => config = new PluginConfig();

        protected override void LoadConfig()
        {
            base.LoadConfig();
            try
            {
                config = Config.ReadObject<PluginConfig>();
                if (config == null) throw new Exception();
            }
            catch
            {
                PrintWarning("Configuration file is invalid, using defaults");
                LoadDefaultConfig();
            }
            SaveConfig();
        }

        protected override void SaveConfig() => Config.WriteObject(config);

        protected override void LoadDefaultMessages()
        {
            lang.RegisterMessages(new Dictionary<string, string>
            {
                ["Blocked"] = "You cannot upgrade past this grade on this server.",
                ["BlockedWithGrade"] = "You cannot upgrade to {0}. Your maximum building grade is {1}.",
                ["YourLimit"] = "Your maximum building grade is {0}.",
                ["NoLimit"] = "You have no building grade limit."
            }, this);
        }

        private string Msg(string key, string userId = null, params object[] args) =>
            string.Format(lang.GetMessage(key, this, userId), args);

        private void Init()
        {
            permission.RegisterPermission(PermUse, this);
            permission.RegisterPermission(PermBypass, this);
            foreach (var perm in config.GradePermissions.Keys)
                permission.RegisterPermission(perm, this);
        }

        private object OnStructureUpgrade(BuildingBlock buildingBlock, BasePlayer player, BuildingGrade.Enum type, ulong skin)
        {
            if (buildingBlock == null || player == null) return null;
            if (config.IgnoreAdmins && player.IsAdmin) return null;
            if (permission.UserHasPermission(player.UserIDString, PermBypass)) return null;

            var max = GetMaxGrade(player.UserIDString);
            if (type <= max) return null;

            var msg = config.ShowGradeInMessage
                ? Msg("BlockedWithGrade", player.UserIDString, FormatGrade(type), FormatGrade(max))
                : Msg("Blocked", player.UserIDString);
            SendReply(player, msg);

            if (config.AnnounceBlocked)
                Puts($"{player.displayName} ({player.UserIDString}) blocked from upgrading to {type} (max {max})");

            return false;
        }

        [ChatCommand("grade")]
        private void CmdGrade(BasePlayer player, string command, string[] args)
        {
            if (player == null) return;
            if (!permission.UserHasPermission(player.UserIDString, PermUse))
            {
                SendReply(player, "You do not have permission to use this command.");
                return;
            }

            if (permission.UserHasPermission(player.UserIDString, PermBypass) || (config.IgnoreAdmins && player.IsAdmin))
            {
                SendReply(player, Msg("NoLimit", player.UserIDString));
                return;
            }

            SendReply(player, Msg("YourLimit", player.UserIDString, FormatGrade(GetMaxGrade(player.UserIDString))));
        }

        private BuildingGrade.Enum GetMaxGrade(string userId)
        {
            var best = ParseGrade(config.DefaultMaxGrade, BuildingGrade.Enum.Wood);
            foreach (var entry in config.GradePermissions)
            {
                if (!permission.UserHasPermission(userId, entry.Key)) continue;
                var grade = ParseGrade(entry.Value, BuildingGrade.Enum.Twigs);
                if (grade > best) best = grade;
            }
            return best;
        }

        private static BuildingGrade.Enum ParseGrade(string value, BuildingGrade.Enum fallback)
        {
            BuildingGrade.Enum grade;
            if (!string.IsNullOrEmpty(value) && Enum.TryParse(value, true, out grade))
                return grade;
            return fallback;
        }

        private static string FormatGrade(BuildingGrade.Enum grade)
        {
            switch (grade)
            {
                case BuildingGrade.Enum.Twigs: return "twig";
                case BuildingGrade.Enum.Wood: return "wood";
                case BuildingGrade.Enum.Stone: return "stone";
                case BuildingGrade.Enum.Metal: return "metal";
                case BuildingGrade.Enum.TopTier: return "armored";
                default: return grade.ToString();
            }
        }
    }
}