The thickest thread I use is Gütermanns polyester extra strength and top stitch, and the weight. I keep forgetting how thread weight numbers work and I have to look it up on a chart every time. Like I mention the large size needles for heavier fabrics have made the difference, like upholstery with coarser thread, twill weave wool, or densely woven cotton duck,... The #21 size Singer needles you have are top quality and the size isn't made anymore so don't waste them.
I think it's the large size eye and the groove on the side of the needle that solves the problem, or maybe just the sturdiness the thicker steel, less bening? The Chinese large size needles are all right, but not quite up to the best. They aren't cut or polished as finely. I have to say I haven't had any problem with the large size needles #20-22, only the thinner. Either way, I'm sure you have the solution right there among your sewing equipment. Gütermann sew-all weight is easy on all types of fabric, it's with the top stitch or fancier threads that's been a bit more of a challenge with bobbin tension and choice of needle.