The vendor neutral standard is the Operating System. You don't need the format of the OS at disk to be standardized, that assumes a wild misconception of OS.
The OS integrates with hardware so you typically can't just copy an image between different machines and expect it to work, what's standard is the installation procedure which can be part OS provided and part hardware manufacturer/host provided.
Yes the process for installing the operating and configuring system is a bit of glue that might not be vendor neutral, but if you are obsessing over that and spending time with stuff like terraform you are missing the forest for the trees. It's like open source zealots complaining that github is closed source. You are 99% of the way there, forget about the 1% last mile and just port it manually.