I don't think adding a granite mix or sand will sufficiently change the resins heat deflection temp to mitigate any potential issues from using low quality resins. Other properties can be changed though. For example, I added chemicals to make it flame retardant and carbon nanotubes to increase stiffness.
I have tried most of the commonly used resin brands and so far Adtech 820 is the best for wet lay ups. It has the highest stiffness and heat deflection temp without any post curing. It's noticeable better than West systems in every way except price. For most people, 180 degrees is probably good enough to use without post curing. For those with curing ovens like me, it can be post cured to 350 degrees giving it similar heat resistance to most heat cure prepregs.
I make my own prepreg these days and keep it in my freezer ready to use. It's far more cost effective and I get better results than most of the bought-in prepregs I have tried. I recommend it you haven't tried it yet. It changed everything for me.
I think that heat curing or post curing large parts, such as people building larger CNC machines, is always going to be an issue for hobby users. I own a composites business so it makes sense for me to have a curing oven. I doubt I would want a large curing oven or hot box in my house if I was just a hobby user. M wife would kill me. I think there are some credible alternatives these days though thankfully.