I would like to remove the mod from my world because I can't use the weapons; it crashes the client constantly. I've tried removing the .jar file, but the server doesn't finish loading and insists on putting the mod's .jar file back. I'm playing on version 1.20.1.

You have manually edit the nether and level.dat to remove any references to the doom structures/dim. This is how Minecraft works past 1.16.3+. You can't remove mods that affect world generation and expect it keep the world the same anymore.