Repeated measures ANOVA on correlation coefficients

The purpose of the study is to examine a gradual linear decay in an EEG signal over a period of time.
I measured the average signal over 20 bins of 50ms each and correlated it with sequential time points, to get negative r Pearson, which would reflect the decay.
I have a 2X2 mixed model (one within-subject factor and one between-subjects factor). Following the analysis I now have an r Pearson value for each subject in each condition.
The purpose is to find an interaction between the two factors.
How do I do that?
I've tried using r-to-z Fisher transform and then performing regular repeated measures. Is this the correct approach? Or since these are dependent samples I need something more?
I also tried permutation analysis but it's complicated for interaction analysis. I'm not sure how to do it.
Would be happy for advice,