Focusing only on the charge rate in relation to cell impact, missing the picture in regards to BMS balancing performance. A lower charge rate would allow BMS balancing time to adjust to differences in module pack differences at finer level. Exactly what the limits of the BMS balancing circuit is, I'm not sure, but generally with the lower the charge rate, the easier it is for a pack to be balanced.
My understanding is that the pack doesn't do much balancing unless you charge it above ~95% anyways.
Either way, if it can do balancing below that, what's to stop you from balancing the pack after it 'finishes' charging? I'm not sure why it needs to be actively consuming current from the wall to balance a battery.