Useful Plugins

WorldEdit 

EssentialX

CoreProtect

ViaVersions - allow other similar version to join the server without conflict

Offline Mode/Mobile Bedrock

To allow offline play for PC version. Change server.properties and edit these lines

enforce-whitelist=false
online-mode=false

Refer to Minecraft Prep and Install to install offline client.

For bedrock compatibility, need the geyser plugin.

Geyser

To allows offline play for bedrock mobile version. Go to ./plugins/Geyser-Spigot/config.yml and change these lines. Do not install the plugin floodgate, if it's installed, removed the plugin. ViaVersions is also needed for mobile play.

auth-type: offline
enable-proxy-connections: true

Now client can play without login to Xbox or Java.

WorldGuard