I think you are on the right track, and agree with others that the single biggest improvement you can make is to have the controller close to the battery.
I built a portable 100w kit, and used 12-2 landscape wire from the local big box home center. I bought 50', and rig it as two 25' sections. So if I am in the sun, I can use only 25' (could have been shorter), and if I need to extend it, I can by using the other 25'. I used Andersen Power Pole connectors, the small ones, and learned how to crimp them with regular crimp tools through youtube.
Since the panel is putting out 18v give or take, you might be down a volt by the time it gets to the controller, but that still gives you plenty for the controller to work with. I used 10 ga wire to the battery, but even the 12-2 probably would have been okay.
I used a Morningstar SunSaver. On cool days, it pushes up near 15v. Probably over 15v on cold days, but I haven't tried it on a cold day. With only 120w, I don't think you'll benefit much by trying too hard to optimize the controller, measure your throughput, and all that jazz. Just get the basics right, IMO.
Here's my thread
