I've no experience with iodine tests (since I don't dose it), so I can't really recommend one. But I've always been happy with Salifert kits.
My concerns are on a couple fronts...
I've read more accounts of people overdosing iodine than any other element, even when they're dosing in amounts far less than directed on the bottle. It's a slow buildup over time, and the coral doesn't use it up fast enough. Here's a good article:
The other concern is the timing of your water changes. Three weeks is kind of pushing it, in my opinion. And with dosing iodine, it might be too little water over too long of time to get rid of the excess iodine. My point is that it could take months to get to poisonous levels of iodine, but you could slowly be ratcheting up the levels.
One of the few cardinal rules in reefkeeping I've found: If you dose it, then you'd better be testing for it.
PS... Nice coral growth, by the way! Obviously, things are happy in your tank right now! I just bring up the iodine issue because it could be an issue down the road if you aren't watching for it.