Fiddling with polygonizer is worth a shot, the polygonizer length will make some difference, but increasing the "shortest vector" will do more - at the expense of losing a little detail - that you probably won't notice anyway.
If you are driving the machine via USB, then vector drawing can be interminable, because the protocol is slow - lots of stops and starts. If it's those stops and starts that's causing problems with bouncing pens, then there's not much you can do about it beyond slowing things down, reducing the acceleration.
The biggest problem with the adafruit shields is that there's no real tuning possible, and I guess it's fair to say that I don't know if there's a better way to match motors, drivers and power, beyond:
Motors should draw around 0.6A because L293D can handle that continuously.
Power supply should be able to supply 1.2A because two motors x 0.6A.
Microstepping is a possible option for you, and it does smooth movement, but I've never got on with it. It's weird and squealy and rubs me up the wrong way. You'd need to recompile the firmware to change it.