Finally getting to trying to do the same thing here, with the hope of being able to dig more into the kernel stuff once I’ve got my own image.
In general I’ve been able to follow your notes, with minor deviations (I’m building on Ubuntu 21.10 22.04 which is what I have handy on a laptop, but that’s Debian-ish enough for the most part…). There are a few notes I could add, specifically as noted here, the python2.7 dependency is ridiculous and can be ignored anyway (the code can be modded to use Python 3 in both devterm-audio-patch and devterm-fan-control-cpi), and there are a few typos to watch out for, but this is otherwise a great guide! Thank you for sharing this!
I’m falling over at the point of trying to get the disk image built and copied to an SD card. I’ll focus just on the bits that touch the disk image…
Everything under Create new disk image file, partition and make filesystems works
Skipping on, the next part where we touch the image is under Compile bootloader and again, those dd commands work without particular concern (barring the fact that dd is dangerous in itself… 
)
So then we build the base Debian setup and everything is looking happy until…
… I get to Copy rootfs to disk image and sd card and this is where I’ve hit an issue. Basically trying to mount the separate partitions in the disk image is failing…
$ d=$(sudo losetup --show -f -P disk.img)
$ sudo mount ${d}p2 disk.img.d
mount: /home/andy/devterm/disk.img.d: special device /dev/loop1p2 does not exist.
$ sudo mount $d tmp/fs/folder
NTFS signature is missing.
Failed to mount '/dev/loop1': Invalid argument
The device '/dev/loop1' doesn't seem to have a valid NTFS.
Maybe the wrong device is used? Or the whole disk instead of a
partition (e.g. /dev/sda, not /dev/sda1)? Or the other way around?
$ fdisk -l disk.img
Disk disk.img: 17 MiB, 17822720 bytes, 34810 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x0b473fe1
Device Boot Start End Sectors Size Id Type
disk.img1 65536 270335 204800 100M 83 Linux
disk.img2 270336 8388607 8118272 3.9G 83 Linux
So it looks like the disk image does contain the two partitions but I can’t mount them to copy data in…
I’m confused by the NTFS reference as well.
Any chance you might have an inkling what I’ve missed here, @thomasdstewart ? 
ICYMI, my own notes and explorations, on the default Ubuntu installation so far, are here 