buzz, it is an LG OLED77W8PUA (it's in the title).


STTtech,
As you have already discovered, there is no way to accomplish what you are wanting to do. There is no factory way to acquire a variable signal anywhere, to run a dedicated sub. Your only recourse, is to install a whole sound system.
I would note that anyone really interested in doing a sound system with an LG OLED TV, would/should not choose the 77" W8 series. As it wasn't really designed with that in mind. You could have gone with the 77" C8 series and lost nothing in picture quality. With the money saved and a more conventional design, it would have been easier to integrate a full sound system. And the results would have still been better than using the W8 sound bar.
Unless the whole 'wall paper' TV thing was the 'cool' factor decision on this - then you have to live with what you get.

There is a way to add a subwoofer to this setup however. but it will void your warranty and not look as nice. Take the W8 sound bar apart, locate the speaker wire going to the internal woofers. Splice in a LOC (Line Output Converter) to give you a variable line level signal to run to a subwoofer.
Sound quality will also suffer a bit going this route, but it will probably sound fine for what you are wanting to do. It is very time consuming and you need to know what you are looking at and understand what you are hooking up. But it can be done.