I batch anneal and make a lot
of lentils; make sure that you 'waft' (I'm sure that's the technical term
) the bead several times through the flame to even out the heat, before counting to five (or whatever you count to!) and putting it in your bubbles/ vermiculite. I used to have the same problem, but with the good old 'ppp', rarely have a breakage now.
my slow cooker is always cold, and I don't heat the bubbles in any way; the few degrees that I would be able to heat it up to would IMHO not be enough to make a difference. If you think, the strain temperature point of soft glass is 454 degrees, much under that and I don't think it's likely to stop cracking, so I don't bother.
Hope this helps.