So if you download this mod as well as essential things seem fine, you make your world multiplayer, things still seem fine, but once someone joins the world the mod simply doesnt work anymore, you can put anything you want into the armor slots and press the buttons to make things visible or invisible but it just doesnt do anything visually even on your end.

Im assuming this has something to do with the way essential lets people join each other and the fact that this mod needs to be on both client and server to function but I have to assume theres some way to fix this. (just a guess though.)

Hmm... my mod sends data packets through network channels provided by Forge/NeoForge.
Without these network channels properly setup, nothing will change as client side don't have any information about the changes user made, as the changes need to be sent to server side and be verified then translated to information that client side needs to know.
My guess would be the network channels got broken during sessions between clients using essential mod...