I'll try to balance the battery by leaving it plugged in the charger for 12 hours for 3 times. From what I understand this could help.
Remember that all balancing means is to make all the cells the same voltage at some point in the state of charge curve, usually at the full voltage point.
It doesn't fix the problems the cells have that cause imbalance--those require replacing the cells with ones that match the remaining cells in capacity, resistance, and all the other properties.
Imbalance is caused by the differences between cells, either because they were mismatched to start with (most common), or because they're old and wearing out.
Batteries that start off with problems or don't have the full capacity they are advertised for, are often made of mismatched cells. That can simply be that they are all new same-model cells from different batches or unsorted same-batch cells. Or it can be as bad as the pack being built ouf recycled garbage cells, which by nature are completely unmatched and often wildly different from each other (commonly not even the same brand, model, and possibly not even the same chemistry), sometimes rewrapped and recapped as if they were some name-brand new cell, sometimes left original.
One question about balancing, is it needed to discharge it to a certain percentage before doing it or it doesn't matter?
Most BMS that have a balancing function (many cheap ones don't) use it only at the very top of charge, when the battery is reaching full.
They usually have a very tiny shunting current ability, so a major imbalance can take days or even weeks to "correct" (keeping in mind that this doesn't fix any problem other than making them all the same voltage; they're still different capacities, etc).
