Steve Si

What ISO? Exactly? Where was it from?

Ventoy can make a GPT USB disk or an MBR USB disk. Ventoy can make a drive with Secure Boot support or without Secure Boot support. So this is FOUR...

Just add the grubfm_multiarch.iso to the first partition.

Did you try VTOY_LINUX_REMOUNT https://ventoy.net/en/plugin_control.html

You can add e2b to a Ventoy USB stick for legacy booting of devuan https://easy2boot.xyz/ventoy/add-easy2boot-to-a-ventoy-usb-drive/

Bios boot or UEFI?

Did you try VTOY_LINUX_REMOUNT which stops the volume being read-only?

Ok, but the documentation does not make this clear at all, it says row and column with no distinction in units?