I would shop around for something like this
https://www.amazon.com/Charger-Battery-Balance-Discharger-Adapter/dp/B07R18YNZQYou want the ability to charge and discharge batteries.
I would suggest watching videos or whatever to discover if it accurately tracks the energy that goes into or out of the battery during charging and discharging. This will allow you to weed out any weak cells that are good, but have less capacity than the average cells. In theory you could wire several in series and use the balance leads to charge a group of cells. Just depends on your goal.
Beyond energy tracking, you want a decent charge rate, more amps is generally good. You can usually configure a lower charging current, so if you want to go slower than rated, you can. That is another feature to confirm though. 10 to 12 amps is pretty good for a hobby charger, and probably fast enough for your purposes.
