I like the build method for your front splitter/airdam. I think it is likely to produce some downforce with that forward and scooping design. It might also lower the press peak point and separation point at the nose? Generally, I think that is considered good. But you would want to try testing to be sure.
That dusty road technique is old, and I think it can reveal useful things. I did some a couple times on the El Mirage Dry Lake LSR site in maybe 2013 and 2019. I find too that morning mist and due settled on my car will blow off in ways that are suggestive, reminiscent of tuft testing results and dust results.
Your results in the pic about suggest a trip line or other separation device might help. You might consider a box cavity type spoiler with side plates like I tested last year. Thee is a very good study on the box cavity that I and others have posted about repeatedly.
Your rear bumper cover seems mostly clear of dust in the middle. Is that correct?
EDIT: I will add this photo from my dust test last summer:
Ridiculous how similar the shape of our cars are. In this shot you can see that without the rear spoiler, dust settled heavily on my bumper cover. When I added the decklid spoiler mockup, this was the result.
