I've used flatworm exit, and it works well. Here's what I would do:

1. Try to remove as many flatworms as possible manually from everything you plan on putting in the new tank. This can be shaking, blowing them off with a powerhead, whatever. I'd limit the freshwater dip to as few things as possible. Maybe, instead, you could remove some water from your tank, put it in a bucket, put a triple dose of the flatworm exit in that water (like 3 drops/gallon) and dip your corals/rock in that for a few minutes.

2. Once everything is in the new tank, treat it with a heavy (double) dose of flatworm exit. The toxicity comes from dead flatworms. If you've removed almost all of the flatworms from the stuff going into the new tank, you should have very few in the new tank. You're not using any of your old sand, are you? And, I'd definitely dip/shake any pieces of rock going into the new tank.

3. Let the flatworm exit work in the new tank for a day, then remove it with carbon under some pressure. Again, you won't have much to worry about if you have done a good job removing the flatworms from the stuff going into the new tank. The treatment is just to make sure there are no survivors to reinfest your new tank.