I have a similar situation as described her
Initial Guess for Trigonometric Curve Fitting
However, now I would like to fit measured data as described in the linked post with a polynoimal of the form
$$y=A_0+\sum_{n=1}^pA_n\cos(n\varphi - \alpha_n)$$ with a possible phase-shift $\alpha_n$ (it could be the case, that $\alpha_n = n \cdot \alpha$ for a constant $\alpha$; this would make sense in the physical context but is not however proofed in any way).
Is a approach as described in the link post here possible as well?