Buffer amplifier, minimizing PCB footprint
This task seems to be high-level analog telemetry.
Given 22 &quot;high impedance, low frequency&quot; signals, why not do this:

a large capacitor on each signal, to remove EMI/RFI; perhaps even R+C LPF; and you may want 2 Clamp Diodes(schottky) across each capacitor, to protect against voltages/spikes/connection_errors to energy outside the range of 0v/+3.3v

3 analog multiplexors, 8:1 that feed into a

4:1 analog mux, 3 channels for the signals and one channel for Calibration/Zeroing.

A single FAST unity_gain buffer, into a

R+C LPF that stores charge for the ADC to sample

The ADC


Notice the ADC needs to take a sample, then begin conversion. At that point, with the ADC's internal Sample/Hld now in HOLD, the entire analog signal chain can switch to another sensor, changing channels on an 8:1 and perhaps  even the 4:1 as needed. Or implement ZERO or FULL_SCALE(VREF) calibration.
There will be capacitive glitches from the gate_to_channel charge movement in the MOSFETS of analog multiplexor ICs. Assume 5 volts and 20 pF, times two because of 8:1 and 4:1 multiplexors may simultaneously switch and inject charge.
If you want 10 millivolt accuracy (you have given no accuracy requirements, so I'll run this analysis with 10 milliVolts), let us compute the injected charge and then compute how large a capacitor must be placed on each sensor output.
We assume the gate_channel capacitance is 20pF.
Q = C * V = 2muxes * 20pF per mux) * 5volts gate_channel voltage step
Notce this is worst case because as one channel is turned on, another channel (perhaps in another mux) will be turned off. Because of lack of guaranteed matching of gate_channel capacitances, we'll just compute worst case.
Q (in coulombs) = 2 * 20pF * 5v = 00 picoCoulombs
Now what size of capacitor to use on EACH OF THE 22 sensor lines?
Using Q = C * V, we re-arrange (use that algebra) for C = Q/V and we have

C = Q/V = 200 (picoCoulombs) / 1e-2 (volts) = 200e-12 / 1e-2 = 200e(-12+2)

C_input_line = 200e-10 = 2e-8 = 0.02e-6 = 0.02 uF


Thus you can install 0.022uF on each of the 22 lines, with guarantee of 10 millivolt maximum voltage upset due to analog Mux switching change injection.
Depending on the &quot;high impedance&quot; of your sensors, the charge may well bleed off before your ADC needs to take a sample of that channel.
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If you want 3X the stabilization time, then cycle thru the 4:1 mux quickly, and over the 8:1 muxes slowly.