Radio magnetars are exotic sources noted for their diverse spectro-temporal phenomenology and pulse profile variations over weeks to months. Unusual for radio magnetars, the Galactic Center (GC) magnetar $rm PSR~J1745-2900$ has been continually active since its discovery in 2013. We monitored the GC magnetar at $rm 4-8~GHz$ for 6 hours in August$-$September 2019 using the Robert C. Byrd Green Bank Telescope. During our observations, the GC magnetar emitted a flat fluence spectrum over $rm 5-8~GHz$ to within $2sigma$ uncertainty. From our data, we estimate a $rm 6.4~GHz$ period-averaged flux density, $overline{S}_{6.4} approx (240 pm 5)~mu$Jy. Tracking the temporal evolution of $overline{S}_{6.4}$, we infer a gradual weakening of GC magnetar activity during $2016-2019$ relative to that between $2013-2015.5$. Typical single pulses detected in our study reveal marginally resolved sub-pulses with opposing spectral indices, a feature characteristic of radio magnetars but unseen in rotation-powered pulsars. However, unlike in fast radio bursts, these sub-pulses exhibit no perceptible radio frequency drifts. Throughout our observing span, $rm simeq 5~ms$ scattered pulses significantly jitter within two stable emission components of widths, $rm 220~ms$ and $rm 140~ms$, respectively, in the average pulse profile.
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