fir filter for fsk demodulator

I had a further look at the design. In fact you can decimate much more.

First your nco should produce the required range and not just for 4050 case.

your worst case is 20KHz signal. you will push it to zero so the other sideband goes to 40Khz which is then filtered followed by decimation.

Hence you can decimate right to cover the 200Hz signal i.e. 400samples per sec or say 1k i.e. decimation rate of 200K/1 = 200.

if you choose this scenario then the filter must cut at 200Hz but sharply at 500Hz instead of 6300 thus it will increase taps but decimation will reduce multipliers by 200

The choice is yours