Lower back pain can be caused by pushing legs and feet behind you, this is done because it shifts the center of gravity forward, more under the holds that you're hanging, to make the hang easier or last for longer.
Remember that you're "training", not performing...
Anything over 40kg added to the front of my harness was seriously hurting my back so I decided to move to one arm hangs instead. Taking weight off and using one arm is much better in my opinion.
Lower back pain can be caused by pushing legs and feet behind you, this is done because it shifts the center of gravity forward, more under the holds that you're hanging, to make the hang easier or last for longer. This goes along with shoulder issues because tehy tend to lock behind your head and neck. As for adding weight, attaching it on the front increases the anterior tilt of the pelvis, which can cause lower back pain due to the extension. So, attach weights on the back or shift to one arm taking some weight off with a pulley.
If you hang with arms bent you will be able to hang from smaller holds. How much smaller depends on your shoulder/core strength.