My charger is one of those that charges the individual cells through a separate connector. It then balances them, presumably to exactly 4.2V, but I am not sure whether anything else is involved in the balance stage. Among hobbyists, series charging of LiPo packs is quickly losing favor because of the problem of over-charging some of the cells.
With respect to the personal anecdote, the charge current was set at 1.0 A, which was well below the maximum allowed rate of 2.1 A for the cells I was using.
My glider, which has had only one flight after conversion from a pure glider to electric was a dream to fly. It soared as well or better than it did before conversion.
You need another sailplane, rather than fight trying to keep some brick up. For the type of gliders I fly, it is not so much weight as it is drag. They can go quite fast, but lose little altitude. John