what is the the resultant, or, "final" frequency of the FFT of a chirped signal
Depending on the chirp, if you seek a closed form solution, there may not be one. For empirical estimation, one can use non-stationary methods.
Take exponential with frequency ranging from 0.005 to .125 of sampling frequency. Applying synchrosqueezed CWT and plotting energy norm per row vs cumulative sum of rows' associated frequency:

The red line marks where energy contributions grow insignificant (and thus max freq is likely reached), which can be found with a flatline detection algorithm. It's heuristics and engineering.
Code below, can be improved.
import numpy as np
from ssqueezepy import ssq_cwt, TestSignals
from ssqueezepy.visuals import plot, imshow

def flatline_after_max(x, count=10, rel_dist=.1, max_frac=.99):
    # pick slightly less than max
    max_idx = np.where(x &gt; max_frac*x.max())[0][0]
    ctr = 0
    for i in range(max_idx, len(x)):
        if abs(x[i] - x[i - 1]) / x[i] &lt; rel_dist:
            ctr += 1
        else:
            ctr = 0
        if ctr == count:
            return i - (count - 1)
    
# define test signal ########################################################
N = 4096  # sampling rate (or number of samples)
fmin = .005
fmax = .125
x = TestSignals().echirp(N=4096, fmin=fmin*N, fmax=fmax*N)[0]

# synchrosqueeze &amp; visualize
Tx, _, ssq_freqs, *_ = ssq_cwt(x, ('gmw', {'beta': 4}))
imshow(Tx, abs=1, yticks=ssq_freqs[::-1], ylabel=&quot;Fraction of fs&quot;,
       title=&quot;abs(SSQ_CWT)&quot;)

# compute row energies and find 'stopping point' #############################
Tx_energies = np.sum(np.abs(Tx[::-1])**2, axis=1)
Tx_energies_cs = np.cumsum(Tx_energies)
max_idx = flatline_after_max(Tx_energies_cs)
est_freq = ssq_freqs[max_idx]

# plot #######################################################################
plot(ssq_freqs, Tx_energies_cs, xlabel=&quot;Fraction of fs&quot;,
     title=&quot;cumsum(energy(Tx, axis=1)) | est_freq=%.3f&quot; % est_freq,
     vlines=(est_freq, {'color': 'tab:red'}))