It looks like you’ve tried a lot of things on the bow to make it quiet. I have a R/D longbow and I notice that it normally very quiet, but the noise I produce with it varies. There are times when I only hear a soft thunk of sorts and other times it’s disturbingly loud. For me, I’m sure it’s not the bow, it me. More specifically it’s my release. When I’m really getting into my back with the draw and release, it’s quiet as a mouse. When I get sloppy with my form, it gets louder.
There’s a very cool thread going here on form. You might want to consider linking a short video of you shooting. Just a thought. All the best, david