PPG analysis beyond HR at rest

I can't say for sure for your case but usually when sitting still the noise from the accelerometer is so high that the result of the fft is invalid. Personally I don't see this as a big issue because the firmware value is used when confidence is high. Which also only happens when the wrist is not moved.

I've also seen cases in the same situation (sitting still after movement) where firmware reports about 40BPM, hrmmar says 100 and the real value was 80.

However yes there is lots of room for improvement.

But surely the point is to modify the heart rate detected by patching the code by installing the hrmmar app ?

The usecase this targets are apps like Heart Rate Monitor , not Health.