I'm using a bottomless portafilter with the Rancilio and noticed that when I do WDT, I use more coffee to get it level to the basket so that I can level it off. This causes a denser puck and when I make the shot, all hell breaks loose (splatters everywhere). I think this is because the puck is touching the grouphead. When I don't do WDT and just level it off, the puck is less dense (tamper goes deeper into basket). I get a better extraction (less spatter) this way.

I ordered a scale to weight my doses, but I think that won't be enough for me to get a flat surface. Does anyone else have this problem?

Do the nickel trick to see if you're overdosing the shot. And yes, you would likely benefit from weighing your doses till you can easily estimate your bean mass in the portafilter, with regards to that particular bean. Big variances between different beans.

I don't sweep, nutate, stockfleths, anything other than dose into the portafilter till its mounded, check the weight, pop on the OE aluminum dosing funnel, WDT, one tap to settle then tamp and go. From grind to locked into the grouphead, it takes me 10 seconds, and it never channels or extracts weird, as long as my grind is right and my beans are fresh.

Some ( much more experienced and more expert than I ) have stated that my particular grinder ( the Malkoenig K30 Vario) doesn't need WDT, that the small soft clumps would disappear in the tamp, but the way I see it, it would take me longer to stockfleths, or do the NSEW sweep or whatever grooming, prior to tamping. So I just stick with what works for me. YMMV.

It says you have a Starbucks Barista, so can I assume that the grinder is a Rocky, thus the "Rancilio" reference? You might want to update your equipment listing in your profile.

But that doesn't matter.

When you do the WDT you should not be putting more coffee into the basket afterwards. You should also be weighing your dose for consistency. How much coffee you are using is very important. Pick up a gram scale that can weigh to .1 grams. You aren't getting a more-dense puck - you are definitely over-dosing. Dose, level off, WDT (if you must), tamp, and be done with it.

A dry day will have the same effect. Humidity can play havoc on dosing. A scale is your best option for consistent dosing. The meltdown shot is often symptom of too much coffee in the basket. The puck will scrape against the shower screen causing cracks in the puck or breaking the side cohesion with the basket.