I have some experience with sparse cheeks and I think you have nothing to worry about. You are only 6 weeks in and to be honest that's not really that long, some guys might have full thick beards at 6 weeks but most guys will not. If you let the cheeks alone for a few months then you'll be amazed at how much they will fill in over time, I don't think you have nothing to worry about, it's looking great so far!