Highly recommended accessories:
* 0.1g scale
* bottomless portafilter
How did you decide that you have a problem with edge channeling? Have you observed it with a bottomless portafilter? (Sorry, can't tell from your pix.) If not... well, puckology is an arcane science. I don't find much value in it. Get a bottomless portafilter, then you will know whether you really have edge channeling.
Even more important: get a 0.1g resolution scale, and start weighing your doses. If you are dosing over 16g in the double basket, then I concur with the downdosing suggestion.
There is no reason why the WDT, done properly, should cause channeling. If you are using a blunt stirring tool (like a chopstick), then try something thinner (like a dissecting needle). If your dosing funnel extends deeply into the basket, trim the bottom of the funnel until it only extends 1/8" into the basket. After removing the funnel, you can give the grinds a little shake to eliminate any possibility of leaving a gap at the edge.
You may wish to try nutation
before tamping. Nutation can improve grinds distribution and help with edge sealing.