How can slope of low-pass biquad be configured?
The decay is determined by the filter order. With a biquad (i.e., second order) filter you can't get more than approximately 40dB/decade (unless you're close to the Nyquist frequency where the frequency response has its zero). You can have less if you like, but not more. If you want less, then the only way to do that is to approximate a given slope by using some optimization method.