OP has Pi 3b+, which doesn't have USB3 ports.
That said, if anyone was building a custom board like this, it would be nice to have some future proofing since USB3 is anyway backwards compatible.
Ah, in that case this board won't cause any further bottleneck. But, to reiterate, what
@Heisen has developed is a USB2 power injector, and won't do anything over USB2.
Using a pi 3b+ as a nas is a terrible idea, because
a. The usb ports are usb 2, crippling your ssd bandwidth. Even if that's not a concern,
b. Even the ethernet port is connected over USB2 internally, max transfers in and out of that nic is going to be ~300mbps realistically
If possible, please upgrade to rpi 4. It has 2x usb3, so even with a usb hub, splitting the bandwidth 4 ways you get 1.25gbps each port. Also, the ethernet is true gigabit connected over pcie, not USB.