I had 15 in the front like yours. Under load the #15 battery had excessive draw when hooked up to the 6 foot cable that ran to the second string of batteries under my rear seat. Dave said he had a problem like that once before with a 3 foot cable. Not sure if mine was a fluke or not. So i moved all batteries to the rear, I didnt feel much if any of a difference in handling. Those batteries weigh a fraction of what a lead acid does.
If I was to do this all over again, I would skip the BMS and go with Dave’s stand alone boards. Top charge with Mike’s suggestion, ICharger X6, then let the boards do their thing along the way. The BMS is nice to view each cell though…