If you managed to blow up a driver at 12v and 1.2A then it just shows how poorly made they are... probably no thermal paste between chip and heatsink.... is this the one? CNC Kit 4 Axis For Nema17 Nema23 Nema34 Motor 5 Axis Breakout Board TB6600HG 5A | eBay

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Running those 8Nm/5.6A motors (which ideally would like 70v drives) at 1.2A & 12v means you're getting 1.8Nm stall torque and very little torque above about 10rpm so beats me why you bought such huge & expensive motors, a cheap Nema23 would have done the job.

What is the intended use?