PCs, especially cheap PCs, have pretty big power supplies. The "850
watt" power supply actually draws over 1200 watts. It sounds like your
PC is simply too much for the system. It may be necessary to downsize
the PC, maybe get rid of performance video cards, get a smaller power
supply, etc. One of the "micro-PCs or a Mac Mini would be a great
That "free" wire is actually a bit of a millstone around your neck in your situation
The problem with your "free" 4/0 wire and quadruplex (4/0-4/0-4/0?-4) is that it's somewhat too large for the job at hand. You see, devices like breakers, switches, and panels can't accept arbitrarily large wires for simple reasons of physical space, ...
There's no magic here. Plenty of engineering, though.
Yes, the inverter output needs to be greater than your peak load.
Not only that, your battery bank size needs to be adequate that the peak load is not exceeding the battery bank charge drawdown rate. Hint, yours is not - you're contemplating drawing over 100 amps at 24 volts off a 200AH battery, or C/2 - ...