The spectrum function has a parameter for setting the data amount. That was already implemented some time ago.

Aside from that: The mere existence of peaks in a spectrum is indeed no proof of cycles in the price curve, since short-lived cycles are in fact found in random data when you look only at a short period. It's the same for almost all inefficiencies. A single 1-week trend, mean reversion, or cycle can be real or just random. For determining a real inefficiency, you must look at a long period of price data and check if the anomalies occur more often than in random data.