The safest and easiest way would be to build another 10S2P (or 1P if the cells can handle your load) and make them samewhat swapable. Because venom is right it’s not worthwhile nor safe to use well used cells with added new cells in the same battery.
As he said the resistance of the cells is drifting (a bit) by time and use, mixing them with new ones will lead to a unbalanced load on the older cells (since the heat “loss”, voltage sag and power delivery are all effected by the resistance) till they can’t handle that load whithout overheating leading to thermal runaway (thus fire or cell venting). The less fatal case would be extra strain on the older cells making them age faster till they die or can’t hold up an enjoyable capacity anymore.
A easy to swap new battery would do the trick, a XT90-s plug or similar and some kind of latching mechanism or case where you but the battery and you are fine. But you have to disconnect the discharged battery when you swap in another one otherwise your full battery will try to charge the discharged one without any Amparege limitation (in case the battery itself has no BMS) which could kill (heat up) both oth them very quickly.