99 Club Car DS - No power up hill

[MightyOakAcresPA]

New member here! I have a 48V club car DS we just upgraded and modified. The cart runs fine on level ground, but struggles when going up hill. If the hill is too steep the cart will stop. Brand new batteries, ran them to a full charge. I tested the total volts after first charge and the reading was in the 90’s. Each individual battery tested at around 15V. Is there a “breaking in” period for the batteries? I tested the batteries under load (while driving on flat, paved ground) and the volts are in the low 50’s. Batteries are brand new, fluid levels in all battery cells are good. Brand new battery wires.
We have it on a new 6” lift, with 22” tires and a new rear seat. The cart acts the same with just the driver behind the wheel or with a passenger.
When putting the cart in reverse I do not hear a reverse buzzer and the cart also struggles when going up a slight incline in reverse.
Any input from you experienced cart folks would be greatly appreciated.