How to make a Rust welcome messages plugin (first-join aware)

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

namespace Oxide.Plugins
{
    [Info("Welcome Messages", "RustPluginGenerator", "1.0.0")]
    [Description("Sends a configurable private welcome message when a player wakes up, with a separate first-join message.")]
    public class WelcomeMessages : RustPlugin
    {
        private const string PermAdmin = "welcomemessages.admin";
        private const string DataFile = "WelcomeMessages/seen";

        private PluginConfig config;
        private HashSet<ulong> seenPlayers = new HashSet<ulong>();
        private readonly HashSet<ulong> pendingFirstJoin = new HashSet<ulong>();
        private readonly HashSet<ulong> welcomedThisSession = new HashSet<ulong>();
        private Timer saveTimer;
        private bool enabled = true;
        private bool dataDirty;

        private class PluginConfig
        {
            [JsonProperty("Welcome lines (returning players)")]
            public string[] WelcomeLines =
            {
                "<color=#ffcc00>Welcome back, {name}!</color>",
                "Server rules are in the F1 console: type <color=#88ddff>rules</color>.",
                "Type <color=#88ddff>/help</color> for a list of commands.",
                "{online} players are online right now."
            };

            [JsonProperty("First join lines (new players)")]
            public string[] FirstJoinLines =
            {
                "<color=#aaffaa>Welcome to the server, {name}!</color>",
                "This looks like your first time here. Read the rules before you build.",
                "You can claim a starter kit with <color=#88ddff>/kit starter</color>.",
                "Discord and wipe schedule are posted in the F1 console.",
                "{online} players are online right now."
            };

            [JsonProperty("Only send welcome once per session")]
            public bool OnlyOncePerSession = false;

            [JsonProperty("Save seen players interval (seconds)")]
            public float SaveInterval = 120f;
        }

        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>
            {
                ["ToggleOn"] = "Welcome messages are now enabled.",
                ["ToggleOff"] = "Welcome messages are now disabled.",
                ["NoPermission"] = "You do not have permission to use this command.",
                ["TestSent"] = "Test welcome message sent to your chat."
            }, this);
        }

        private void Init()
        {
            permission.RegisterPermission(PermAdmin, this);
        }

        private void OnServerInitialized()
        {
            try
            {
                seenPlayers = Interface.Oxide.DataFileSystem.ReadObject<HashSet<ulong>>(DataFile) ?? new HashSet<ulong>();
            }
            catch
            {
                PrintWarning("Could not read seen players data, starting with an empty list");
                seenPlayers = new HashSet<ulong>();
            }

            saveTimer = timer.Every(Mathf.Max(10f, config.SaveInterval), () =>
            {
                if (dataDirty) SaveData();
            });
        }

        private void Unload()
        {
            saveTimer?.Destroy();
            saveTimer = null;
            SaveData();
        }

        private void SaveData()
        {
            if (!dataDirty) return;
            Interface.Oxide.DataFileSystem.WriteObject(DataFile, seenPlayers);
            dataDirty = false;
        }

        private void OnPlayerConnected(BasePlayer player)
        {
            if (player == null || player.IsNpc) return;
            if (player.net?.connection == null) return;

            ulong id = player.userID.Get();
            if (!seenPlayers.Contains(id)) pendingFirstJoin.Add(id);
        }

        private void OnPlayerSleepEnded(BasePlayer player)
        {
            if (!enabled || player == null || player.IsNpc || !player.IsConnected) return;

            ulong id = player.userID.Get();
            bool firstJoin = pendingFirstJoin.Contains(id);
            pendingFirstJoin.Remove(id);

            if (!firstJoin && config.OnlyOncePerSession && welcomedThisSession.Contains(id)) return;
            welcomedThisSession.Add(id);

            string[] lines = firstJoin ? config.FirstJoinLines : config.WelcomeLines;
            SendLines(player, lines);

            if (firstJoin && !seenPlayers.Contains(id))
            {
                seenPlayers.Add(id);
                dataDirty = true;
            }
        }

        private void SendLines(BasePlayer player, string[] lines)
        {
            if (lines == null) return;
            foreach (string raw in lines)
            {
                if (string.IsNullOrEmpty(raw)) continue;
                SendReply(player, Format(raw, player));
            }
        }

        private string Format(string line, BasePlayer player)
        {
            if (string.IsNullOrEmpty(line)) return line;
            return line
                .Replace("{name}", player.displayName ?? "player")
                .Replace("{online}", BasePlayer.activePlayerList.Count.ToString())
                .Replace("{id}", player.UserIDString);
        }

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

        [ChatCommand("welcome")]
        private void CmdWelcome(BasePlayer player, string command, string[] args)
        {
            if (player == null) return;
            if (!permission.UserHasPermission(player.UserIDString, PermAdmin))
            {
                SendReply(player, Msg("NoPermission", player.UserIDString));
                return;
            }

            SendLines(player, config.WelcomeLines);
            SendReply(player, Msg("TestSent", player.UserIDString));
        }

        [ConsoleCommand("welcomemessages.toggle")]
        private void CcmdToggle(ConsoleSystem.Arg arg)
        {
            if (arg == null) return;
            if (arg.Player() != null && !permission.UserHasPermission(arg.Player().UserIDString, PermAdmin))
            {
                arg.ReplyWith(Msg("NoPermission", arg.Player().UserIDString));
                return;
            }

            enabled = !enabled;
            arg.ReplyWith(enabled ? Msg("ToggleOn") : Msg("ToggleOff"));
        }
    }
}