1. XT90 as a connector doesn't make sense. You should use screw terminals. Simpler, inexpensive, reliable. 400W builds is what require a pcb, below that you are fine without the pcb. XT requires soldering and is about 5x the cost of cheaper screw terminals.
2. That mosfet is specced too high, and should be an SMD type.
3. You will need a controller for the mosfet.
4. Tiny leads can push a lot of current because the distance is fairly short. Current also depends on surface area so properly soldered it has plenty of surface area.
5. I would allow for 10C since the ambient temperature would be fairly warm inside a case.
You are essential designing a solid state relay. I don't think we have an electrical engineer on the forums so I recommend you check an EE forum to properly design it.
I also intend to design a solution but it is not on my radar until next year.
I have already designed a board with more features than this called the Distro 400. It is our forum and has all the specs one would need + more.
