Thanks for the help! Just to be clear, the official RGB cable for the GameCube works perfectly on my Super Famicom (regular version, not 1CHIP) and my modded NTSC N64 (the one posted above). Also the guy from eBay has been doing doing the RGB N64 mod for more than 7 years. If there was anything wrong with it, I’d think he’d have figured it out by now (I hope). Will look into buying the NTSC luma sync RGB cable if I face any issues.
I’ll get back to you once I get my OSSC, and will post the results.