It is hard work to make a top of the line high performance computer quite. A more simple solution is actually try to dampen the sound as much as possible.
If there is not yet sound-absorbing material in your case door. Invest in it before doing anything else. It will not quite the gtx by much, but it's a cheap way to get the case just slightly less noisy.
Another thing you can consider is moving the whole machine to another room or closet, then using a 5meter hdmi and 5meter usb to your desktop. (5meter is usb and hdmi limit). If you consider all the work of watercooling that is actually cheap. If you have the space to do it.