AWS offers Graviton instances in a bare-metal way too, so you can see for yourself what the SoC offers. It's quite clean, there is a Designware PCIe filtering quirk (same as on e.g. the Marvell Armada8k) but other than that it's SBSA-style UEFI+ACPI. Very standard system.
In terms of firmware openness, yeah, you don't get the firmware source, but you don't get to touch the machines physically either so you wouldn't have a chance to run a custom build anyway :D
(While Ampere Altras are supported by upstream EDK2, have fun)