Before sending your Surface Pro 9 for repair, I would suggest checking a few things with Microsoft Support.
Since you suspect that the motherboard may be faulty and the repair cost is $600, it’s reasonable to ask whether this amount will guarantee a working device. If your Surface cannot be repaired, will Microsoft replace it with a working unit?
Also, ask what warranty will be provided on the repaired or replacement device and whether any additional charges might apply.
I’d recommend getting these details confirmed in writing before sending your device or paying the amount, so you know exactly what to expect.