Yes, the plastic issue is very relevant these days.
With Clean + easy you save wax though because when a cartridge is part used you pour this into another part full cartridge, so at least you don't waste wax.
On the back of the C+E heater there is a bracket where you can store spare cartridges ready to go. The warmth from the heater keeps these spare ones half heated up. It would take approx 20 mins to get these to full temperature. So you could quickly do a change over while your client is getting off the couch and they would be ready for the next client.
However when I used to do waxing back to back at a fast pace, I found it much simpler to have a second heater on the shelf below full of hot ready to use wax. This way you just swapped over hot wax into your using heater and refill the second heater as you go. On a mega busy day, you just keep back the part used cartridges and when you get quiet time, reheat them and top up again.
Having a second heater is also reassuring in case one heater should ever die on your suddenly!
I think that makes sense!