• there is no SR.function that gets the coefficients of SR.series; rather, coeffs etc give confusing results; truncate must be prepended manually. This is trac #17399.
• even if truncated, the polynomial cannot be converted to a PowerSeries(SR) or Polynomial(SR). The whole symbolic expression is taken as a constant. Your usage of x for everything disguises this. This is trac #16203.
For the problem with simplify_full I have opened trac #17400 but I'm not sure if that ticket is really separate.