Hi All
Hopefully this helps someone else out.
I had the exact same issue as the original poster.
The first layer down on the cool plate supertack, was very uneven and wavy.
No matter how much I cleaned the plate with HOT water and good dish soap or 100% Isopropyl, it would not fix the issue!
After a lot of trial and error of messing with settings I could still not work out how to get this fixed.
Then, I decided to install “Orca Slicer” and do a filament calibration.
What I did was run the “Flow Rate - Pass 1”, and I watched the printer very carefully during this process and noticed that the least extruded test “-20” looked PERFECT on the First layer it put down, and all Flow rate levels above this started to exihibt the same problems. However, the problem was that the rest of the layers would be perfect at a Flow rate ratio of “0” (based on the test plates printed in pass 1, not the filament flow ratio setting 
)
This then told me my first layer flow rate was too high! but the rest were good.
Doing some searches, whether it was possible to lower the flow rate only for the first layer, I found this post: Overextrusion on first layer
You can actually lower the first layer flow rate!
The only thing is that its only possible in the Orca slicer (the setting is not available in Bambu Studio).
The setting you are looking for is “Bottom surface flow ratio” in the “Quality” section with “Advanced” turned on.
The value I used was “0.81” , which resulted in the first layer coming out silky smooth!
The first layer flow ratio is calculated by multiplying the “Bottom surface flow ratio” value with the filament flow ratio (in filament settings)
Long story short, the lower you set the value the lower the flow rate. Do some testing to find your optimal value.
For reference I am using Bambu Lab Basic PLA and the filament flow rate is set at the default of “0.98”