DC/DC convertor max capacitive load
It depends on the DC-DC converter. Can you use an other one? I think it is common that it can supply the peak current and that there is a thermal shutdown that can shut it down.

Why do you need 900µF ? When you need a buffer, the capacitor can be placed at the input of the DC/DC converter. When the converter can supply enough peak current, there is no need for a large capacitor at the output of the DC/DC converter.

When you need to charge a large capacitor, then some kind of current limiting can be used. A voltage regulator or LDO can do that. But when you need a lot of current the LDO heats up and reduces the current.

Is there something wrong with your application ? Perhaps a LC filter might help. Perhaps an other design/circuit will prevent this problem.