I'm getting closer to it, I've trimmed the following (what I think is relevant) from the log:
Sep 16 16:43:17 main-menu[2810]: INFO: Menu item 'grub-installer' selected
Sep 16 16:43:17 grub-installer: info: architecture: amd64/generic
...
Sep 16 16:43:17 in-target: grub-common is already the newest version (2.02+dfsg1-20+deb10u2).
Sep 16 16:43:17 in-target: grub-common set to manually installed.
...
Sep 16 16:43:18 in-target: grub-common is already the newest version (2.02+dfsg1-20+deb10u2).
...
Sep 16 16:43:18 grub-installer: info: Identified partition label for /dev/sdb3: gpt
Sep 16 16:43:19 grub-installer: dpkg: warning: ignoring request to remove grub which isn't installed
Sep 16 16:43:19 grub-installer: dpkg: warning: ignoring request to remove grub-legacy which isn't installed
Sep 16 16:43:19 grub-installer: dpkg: warning: ignoring request to remove grub-efi which isn't installed
Sep 16 16:43:19 grub-installer: dpkg: dependency problems prevent removal of grub-efi-amd64-bin:
Sep 16 16:43:19 grub-installer: shim-signed:amd64 depends on grub-efi-amd64-bin.
Sep 16 16:43:19 grub-installer:
Sep 16 16:43:19 grub-installer: dpkg: error processing package grub-efi-amd64-bin (--purge):
Sep 16 16:43:19 grub-installer: dependency problems - not removing
...
Sep 16 16:43:19 grub-installer: Removing grub-efi-amd64-signed (1+2.02+dfsg1+20+deb10u2) ...
Sep 16 16:43:19 grub-installer: Removing grub-efi-amd64 (2.02+dfsg1-20+deb10u2) ...
Sep 16 16:43:19 grub-installer: Purging configuration files for grub-efi-amd64 (2.02+dfsg1-20+deb10u2) ...
Sep 16 16:43:19 grub-installer: Removing grub-efi-ia32-bin (2.02+dfsg1-20+deb10u2) ...
Sep 16 16:43:19 grub-installer: dpkg: warning: ignoring request to remove grub-efi-ia32 which isn't installed
Sep 16 16:43:19 grub-installer: Errors were encountered while processing:
Sep 16 16:43:19 grub-installer: grub-efi-amd64-bin
...
Sep 16 16:43:19 main-menu[2810]: (process:15530): Volume group "sdb" not found
Sep 16 16:43:19 main-menu[2810]: (process:15530): Cannot process volume group sdb
Sep 16 16:43:19 main-menu[2810]: WARNING **: Configuring 'grub-installer' failed with error code 1
Sep 16 16:43:19 main-menu[2810]: WARNING **: Menu item 'grub-installer' failed.
Sep 16 16:43:19 main-menu[2810]: INFO: Modifying debconf priority limit from 'debconf/priority doesn't exist' to 'low'
...
Sep 16 16:43:20 main-menu[2810]: WARNING **: Internal error! Cannot find "debian-installer/main-menu doesn't exist" in menu.
Sep 16 16:43:20 init: process '/sbin/debian-installer' (pid 2794) exited. Scheduling for restart.
Sep 16 16:43:20 init: starting pid 15850, tty '/dev/tty0': '/sbin/debian-installer'
Sep 16 16:43:21 init: process '/sbin/debian-installer' (pid 15850) exited. Scheduling for restart.
It would appear somehow I'm installing grub wrong, I think.
-SG