Are you still using 12 volt as power supply with series connected steppers?
Not shure how it works with steppers but normally if you put something in series the voltage drops in half and in this case 6 volts instead of 12 volt.
My steppers also have a max of 2.6 amps (4.8kg)
Also planned to use 1 drive for each stepper set at about 1.75 amp.
Another thing that could also be a possibility is run at a higher voltage like 24 volts, haven’t researched it yet with ramps but it was in the back of my mind.