\$\begingroup\$

You should learn what's realistic and what's not, and know your limitations.

You come up with a price of 400 - 500 dollar for a couple of PCB's, but you don't seem to realize that these are just the PCBs, without any components, and without the cost of the design (which you are going to do). That's what an 8 to 12 layer PCB the size of a motherboard costs. You don't know that.

You don't know what components or subsystems a motherboard contains, you only know its mechanical dimensions (fit inside your DIY laptop). You have no clue about design tools: DesignSpark doesn't even get you started. You don't know why that is.

You don't know how to start a high-speed design, and what kind of pitfalls you'll encounter. I mentioned impedance matching, you don't know what that is and why it's important. You don' know what a transmission line is.

You want to breadboard the design, but you don't realize that you can't even make the system work that way, not even at 100MHz, let alone 4GHz. If you would be able to make a digital design at 100kHz you don't know how to scale it to 4GHz.

You don't know that none of the components and subsystems is fit for breadboarding. (Do you know what an LGA is?)

Do I need to go on?