I have had a tough time with Parodia leninghausii’s light needs. They crisp for me in even a few hours of direct sun. Even bright grow lights bronze it’s bottom. I spent time moving things around the space so another plant gave it some shade - in order to create some kind of fake natural environment. I gave up because I assumed I had stunted its growth. Guess what? I put it in a south-facing window that gets bright light or direct sun and had (relatively) exponential growth in a week. That was always the answer ha.