Managed to make some small progress by quickly mashing the keyboard before the system boots so that I can edit the kernel command-line arguments and remove console=ttyS0,115200. This allows the emergency shell to appear on the Vultr console:
I’ve yet to get kernel_arguments to actually remove the arguments in the Butane configuration though.
Yup, that’s the emergency shell failure mode when a serial console is enabled but not present. What version of Fedora CoreOS are you using? Current releases shouldn’t be enabling serial console by default on Vultr.