Filter design for an unusual EEG experiment

I'm designing a quite unusual (in terms of signal analysis) EEG experiment, which forces me to design my own filter. I have never done this before, so kindly please check my work and suggest corrections.

The analysis consists of cutting a narrow frequency band (2Hz) from 6-second EEG recordings(4*) (sampling frequency 256 Hz). I expect that after such filtering, I will receive a "somehow oscillated" signal (not in a strict mathematical sense, just with notable local maxima (peaks) and local minima (troughs))(5*). Now I'm interested just at the very end of this 6-sec window, the last dozen or so data points. I want to determine (by the simple algorithm provided by myself) if the signal ends "near the peak" or "near the trough" or in either of the above. So, all 6-sec windows are compressed to one of the three categories. You can think of this as a very rough estimate of the phase at the end of the window.

The tricky part is that I need to analyze overlapping frequency bands ( 4-6 Hz, 4.25 - 6.25 Hz, 4.5 - 6.5 Hz, etc.), so it will be great to have an extremely steep roll-off even at the price of amplitude disturbance (most of this information will be lost anyway) - that's why I came up with an elliptic IIR filter. Zero-phase shifting is also crucial so, forward-backward (filtfilt) filtering is applied. However, the devil is in the details (filter order, maximum ripple, minimum attenuation), and I must honestly admit that I determined these parameters by trial and error.

from scipy import signal
from mne.viz import plot_filter

sfreq = 256.
f_s = 6.
f_p = 8.
flim = (f_s - 1.0, f_p + 1.0)  # limits for plotting

nyq = sfreq / 2.  # the Nyquist frequency is half our sample rate
freq = [0, f_s, f_s, f_p, f_p, nyq]
gain = [0, 0, 1, 1, 0, 0]

ftype = "ellip"
order = 8
sos = signal.iirfilter(order, [f_s / nyq, f_p/nyq],
btype='bandpass', rp=5, rs=35, ftype=ftype, output='sos')
plot_filter(dict(sos=sos), sfreq, freq, gain,
f'{ftype} order={order}', flim=flim, compensate=True)


Filters Visualisations:

So my questions are as follow:

1. Is my filter "reasonable"?
2. Can I do better?
3. Can you recommend to me some learning materials for a filter design.
4. (*) This is obviously cut from continuous EEG signal - can it be just cut (boxcar window), or should I apply something different (eg. Hamming window)?
5. (*) Is my expectation justified?

Is my filter "reasonable"?

I would say no. The time domain ringing time of your filter far exceeds the length of your signal, there isn't enough signal for the output to stabilize.

Can I do better?

Difficult to say. Filter design involves complicated tradeoffs between many different factors and the "best" really depends on the specific needs & sensitivities of your application. You have partially explained what you want to do but not in enough details to give a recommendation. I'm skeptical about "looking at the phase" approach. Signal phase has only meaning if you have a well defined time reference for $$t=0$$, which most signals don't.

Can you recommend to me some learning materials for a filter design.

I personally like that one https://ccrma.stanford.edu/~jos/filters/ but I'm also mostly and audio guy.

(*) This is obviously cut from continuous EEG signal - can it be just cut (boxcar window), or should I apply something different (eg. Hamming window)?

Depends a bit on your application details. Ideally your filter "ringing" is significantly shorter than your signal. Then you can just take a "large" chunk of signal, filter it and cut off the edges to remove the filter transients.

(*) Is my expectation justified?

Can't say without looking at your signal first. A sufficiently narrow band signal will look like an amplitude and/or phase modulated sine wave. However, if your original signal is very noise-like, you'd have to make it VERY narrow to look like that.