Pico caps and appropriate impedance miss-match can be used to round-off square waves.

But yes, if you look at high-speed Pi GPIO with a Rigol it looks more like an EKG readout than the thing you might see on a logic analyser. Smoothing it enough to feed a line-amp is very lossy.

You might want to improve your probing technique, then. :)

GPIO's usually looks quite squarey if you don't introduce parasitic reactances into the circuit with your test setup.

Pi for audio frequencies is lovely and square, Pi at radio frequencies has distinct rise and fall and "just taking a moment to think about it" segments.

A spectrum analyser has probes!?!? This might be where I am going wrong... But the bench scope is largely in agreement about the distinct phases of a cycle at RF freq.