For multiplayer, use a vanilla server. Spigot/paper/others tend to mess up the dimensions so you may spawn in regular nether. Manual tweaks will be needed.

Ensure Spawn Protection is turned off.

You'll need to make sure the resource pack is being used. This is automatically loaded for singleplayer, but for multiplayer you need to copy 'resources.zip' from the world folder into your resource packs folder and load it in-game.

I balance for 3 or less players. For 4 or more, you may need to farm and grind resources more.