On/off control may be good enough at maintaining set point for this application or it may not - and you'll need to PWM the SCR to achieve power levels in between the full output levels. If you already have SCRs on there then that would lead me to think that they were needed (why would they have paid to put them on otherwise)
Rather than switching every degree - use a PID loop using proportional only control, at least this way if you get to the point that the process needs integral (to correct error over time) then you don't need to reprogram anything
Ignoring deadband - if you want the full output to be on at 15 deg from set point this is a proportional band of 15, or a gain of 6.667 (100/15) etc.
Assume you have 6 * 2kW stages, 100%/6 = 16.667%. So stage the elements on the output from the PID loop so that every 16.67% the next stage kicks in