Try using Box for the background, that’s just a rectangle that shouldn’t interfere with slider responsiveness.
Your layout questions are just solved with more nested GridLayouts. One column with the slider labels, one column with the sliders and their additional labels in rows below. So 4 rows and 2 columns, with halign = :left for the two front labels and halign = :center for the other two.
And for the labels you just do Label where the input is lifted from a slider value attribute. Like Label(..., @lift(string($(slider.value))))