Are you both using precision baskets?
I only use my bottomless PF, mostly for ease of tamping, cleaning, more clearance for cups and rarely have any significant spritzing (I've got white counters too so would definitely see). I don't use a WDT either, I'll stir the grounds in the Niche cup before transferring, levelling (manually, not a tool) and tamping. Surprised, especially with your Niche that you still get a lot of it.
VST basket.
The problem isn't so much about the lumps. I put the dosing cup into the portafilter then shake it like a cocktail, well, not that extreme. But when I remove it, you get.
1 - a ring of clearance on the edge - that's a massive channel
2 - often one side has more ground than the other side
So I would level it with my hand or tap it down. This "tapping" i find is problematic. If I tap it from the side, the forces, just pure physics, pushes the grounds sideways, meaning now one side has more grounds and one edge has a massive clearance on the edge.
So I tap it from underneath? Which is impossible to do that with a single sprout, literally impossible. I know I could with the naked portafilter but i only use it to test how the shots are coming out, I really dislike using it daily. It's good for visuals but not practical for the potential messy reason.
I don't like the double sprout because some cups I have are too narrow for it.
The WDT allows EVEN distribution above everything else. This is even without using the distributor, which I also have and use.
PS I think you should try using a white kitchen towel. I have a white counter too but the white towel shows it up more.