Need a mod to make a sim invisible

There's www.nexusmods.com/thesims4/mods/2267 which can make the sim or object invisible.

Creating a CAS part with many exclude part flags should also work. Wearing 'my precious' would hide the whole Gollum.
In case you want to do it within S4S and clone a ring check sims4studio.com/thread/10175/excludepartflags-information to set the flags.
You'd still need to create at least one outfit with the ring and switch to it in-game.