It's frustrating tracking this stuff down. Sometimes it's good to walk away for a week or two and come back with a fresh perspective(difficult, believe me I know).
If you part with it now, you'll be in the hole and then starting over with zero'ing out another rig.
I chased a vibration in a K5 that I never did find, I was the only one who could feel it, but it was there. Changed ALL of the ujoints, balanced the shafts, it had new tires, wheels were checked, etc. I ended up parting with it, and made good money, but if I would have kept it an extra year into the covid-era prices, I could have doubled my money on it.