My 5 cents again… It’s highly likely that SCART cable is unbuffered, hence not optimal. I had troubles with SCART cable and any other mode than PAL. For some reason PAL worked, others did not. Your VGA adapter, I do not know if it’s buffered or not, but buffered VGA adapter helped a lot for me. But since you have NTSC machine, that adapter from retro68k does not work with them.
Also I think you are expecting too much. Most likely there will be vertical banding always and nothing you can do about it when using external scandoubler. Keep in mind that what you have there is consumer grade product from early 80s made as cheap as possible – no way video out would be without analog artifacts. In real life, in real use cases, vertical banding most likely goes unnoticed. I only notice it when screen is black and stationary.
Noise patterns, I think it has something to do with sampling phase algorithm which is not perfect in the OSSC or something like that. There were some discussion on the other forums about it. When you use “wrong” H. samplerate there won’t be noise anymore because OSSC no longer syncs perfectly to outputted pixels. Again, when you set samnpling phase correctly, noise is hard to notice in real life scenarios.
Of course I am talking about my Amiga only, if yours have more visible banding then it may be different case.