Got my luma cable from Retro Gaming Cables today. The result: the Yoshi’s Island intro now works with the default (sync Vth 123), and for Donkey Kong Country I only need 191, instead of 225 like before, so I can confirm that the cable seems to make a difference, and can potentially fix the issue. A little weird was that on the first try I got static right away from Yoshi’s Island, with no sync at all, but who knows what that was, it worked after testing DKC.
I’ll leave sync Vth at 191 for now, and I’ll report if that causes any issues. I’ll hold off on buying a PACKAPUNCH unless I notice any problems, since I’m pleased with the image I get through the new cable. Thank you guys, I had already ordered the cable, but I didn’t think that the problem might originate there^^
In case it’s of interest to anyone, I’m usually using a cheap, like 10€, SCART switcher for convenience (no noticeable quality difference from 1-2m away), and connecting directly to the OSSC didn’t make a difference in the required sync Vth values.