That's the theoretical max, actual max is around 3400MB/s, should be similar to the 9300-8i in that situation, still more than enough for 8 HDDs.
Yes, just keep in mind that those slots go through the DMI, together with the onboard SATA ports/most M.2 slots, so bandwidth is shared, unlike the x16 CPU slot and M.2 slot 1 that go directly to the CPU, but since DMI for Z690 is PCIe 4.0 x8 it should be fine for all you need.