another thing, while you already have the fir filter there, use it to adjust the center frequency so the sdr isn’t receiving the signal in the center where the dc spike usually lurks. documented for example on the excellent pysdr site. satnogs also uses this method in the flowgraphs.
what is with the odd sampling rate btw, 520833 ? it is the lowest the device can do, but I don’t think that is a particularly good choice. it can be chosen to make the filtering/decimation easier, but I don’t know exactly how really. satnogs usually runs 1-2Msps and it runs fine on small rpi’s so selecting 1Msps will probably work just fine.
designing the fsk/gfsk/gmsk demod properly will make demodulation a lot easier. have a look at gr-satellites fsk_demod and use that after the fir filter in your flowgraph.
regarding deviation, measure it, don’t rely on claims or documentation (:
it is basically looking at the floats after the QD, I made some thread on how to do it and visualize.