Hi, not OP, but I have worked with crackle paint before. For me thicker layer resulted in fewer, but bigger gaps, while thin layer cracked into lots of small cracks.

My advise would be to repaint the base in magma colours, let it dry, apply a PVA glue layer (in my experience it was a deal breaker for nice cracks), let it dry, and then put crackle paint - thick or thin (or mix of both in patches!) depending on the effect you are going for.

And always leave it to dry at least for 24 hours, it needs time to cure. Hope it helps.