Thanks John, very helpful!
The most likely explanation for reduced flow with WDT is elimination of channeling. Any change in your espresso workflow will probably require re-dialing in your extraction. Since you're getting dead spots in your puck, you need to lower your dose and/or coarsen your grind.
That in particular helped me out conceptually. I was stuck thinking about grind quality, after experimenting with sous vide beans, where (I think) more unimodal grind -> less fines -> faster flow. But you're totally right, since with WDT we're not removing any fines, a better distribution of fines in the puck should lead to a more well-restricted flow as a result of less channeling.
I did try doing as you suggested, coarsening my grind. (I'm already at 16g in an 18g basket, so don't really want to lower dose further.) The first two shots I got were gushers, like 60g out in 35s including a 13s preinfusion. They didn't show bald spots, since the flow was heavy, but I did see significant and persistent sprites, which is another sign of channeling and one I haven't seen in a while.
So I tightened my grind back up to roughly where I began. I did WDT as I had been and got a solid shot with minimal/acceptable balding in the last ~3-5s and good taste. Not perfect, but definitely an improvement.
But -- I did change one other parameter with this shot (bad science, I know): my leveling tool was set too deep, since my prior bag of beans was denser, so I adjusted that to just barely level the surface. Since I didn't really change anything else significant with the puck prep process, I wonder if this was the source of my issues; now that I think about it, I switched over to this bag right as I got the collar that coincided the start of my issues. Maybe the leveling tool was too deep in the beans, and was dragging most of the puck around and half-tamping it, destroying my (optimistically good) distribution work from the WDT tool. I'll have to pull some more shots soon to validate.
Don't just pull one sink shot and leave it at that. Fix the mechanics of your extraction, play around with it for a bit, and then draw your conclusions.
For whatever reason I rarely dump my bad shots, I just choke them down. Probably because hand grinding is a pain. I'm definitely always thinking about technique and making small adjustments to process parameters, but iteration happens more slowly. So I took your advice here as well and dumped my two poor shots down the sink! The third one was the charm.
It could be the machine, but I haven't had a single gusher since starting with my Londinium R
Well, let's not rule this one out yet! I'm working with a Gaggia Classic with a meCoffee PID and a few other mods. Certainly serviceable, and I've pulled some damn great shots, but quite inconsistent and lacks real pressure profiling capabilities. I wouldn't be surprised if the machine was causing me a ton of issues I've been compensating for through puck prep, etc. Same with my Lido ET, which is adequate at best and which has a tenuous alignment. Thankfully my Monolith Flat is on the way, and I'm struggling to resist (but not too hard!) buying a DE1+ when they roll out again.