Yes.
Charge them individually, and watch them over their life to see if either starts to discharge and charge differently to the other.
Replace cells that can’t keep up.
And all the other precautions you are running into.
Tip: spare a pin on the microprocessor to measure the battery voltage and alarm or take action if you discharge below, say, 3.3 volts per cell under load.
That they really don’t like…
Also watch that cheap charger and flunk it if it ever goes over 4.2 volts per cell, or be there to remove cells when it is about to.
AND if you do god with 7.4 volts, and a 5 volt Arduino, consider using a buck regulator to give you the 5 volts, your battery will last longer if you get a decent one. See
and go conservative on the specs, test with the load you expect.
a7