Note that the 24-12V DC-DC converter leads are probably only 12 AWG. And the 40A is the output on the 12V side. 6 AWG wire is overkill and it's awkward connecting 6 AWG to 12 AWG. I have a 24V setup and use a nearly identical DC-DC converter. I kept the 12 AWG from the positive bus bar to the converter with a 30A ATO inline fuse on the positive lead. I added 10 AWG on the 12V output leads to the 12V fuse box. I kept all of those wires short - all under 2 feet. This all assumes you are using high quality, pure copper, 105ºC insulation wire.
Since you only show a 12V fuse box and not also a 24V fuse box, why are you setting up a 24V system? In my 24V system, I used as many 24V loads as possible such as the fridge, water pump, all of the LED lights, etc. I added a 12V fuse box with the converter for things that required 12V such as roof fans, 12V outlets, etc. But if you only have 12V loads, you can avoid the DC-DC converter and you probably have a lot more inverter options at 12V.