using buck converter
It says that the input voltage is a minimum of 7 volts and this means that it won't work correctly if the input voltage drops below 7 volts. Don't ask what might happen if you do run it at 6 volts because I won't be able to answer you. There are devices out there that should do what you want. Keep looking.

It also says that the peak current that can be delivered is 3 amps but the wording implies to me it can only deliver 1.5 amps continuously.

Here's a design from Linear technology that can do what you want with what looks like apparent ease: -



There is also this device too: -



And this: -



So, in conclusion, there are designs around and maybe you can find someone vending this as a finished PCB. There are similar offerings from Texas Instruments as well.