You have listed them all. However, it doesn't have to be a gennaker type with the anti-torque built into the luff of the sail, you can install a top down spinnaker furler which can be used with a full spinnaker. They are pretty pricey, though. One such is the profurl spinex here's a video
https://www.youtube.com/watch?v=mo5l4r2N14gI've used top down furlers. I've used anti-torque in luff spins. I've used snuffers. My preference with small boats is snuffers. The only disadvantages of a snuffer are the space it takes and the fact that the larger they are, the more ddw you have to drive to snuff them. A 21 is probably just large enough to make a spin furler worthwhile, but you have to know that there will be two additional lines extending the length of the boat to accommodate the continuous furler-you might be able to get them out of the way with a bunch of blocks to route them around the outside, though.