Fellow Interceptor user here. Don't worry, this is not a question, it's the answer I came up with ;)
TLDR: check my project on GitHub !
Intro / motivation: I was having the same dilemma as some of the other users about preparing a custom distro for the Interceptor board (preferably, a minimal install, as I use mine in a headless setup), especially since I didn't know what the official Interceptor OS is doing (no source code / scripts provided... call it paranoia / I also teach a security course at my university and... "this is the way" :D).
I managed to do this (even automated it!) using debootstrap in a chroot with qemu-user-static / binfmt cross-installation technique (check out the source).
Next, since I plan to use my Interceptor as a custom-made NAS device (5 x SATAs, h3ll yeah), I wanted to use full-disk encryption to provide data-at-rest security. And do this without storing the in unencrypted form (e.g., on the boot partition)... cryptsetup / LUKS FTW, but I also want the device to be accessible when I am away from it (able to remotely enter the decryption password) in the event of a power failure => need to have networking and dropbear SSH server inside my initramfs.
This is where I've been having some problems, which I finally solved (I made an initramfs-tools hook to include swconfig + switch-related kernel modules + udev scripts to enable the WAN port during early boot phase).
I also created an automated kernel building script for completeness, you can use it to build the latest .deb kernel (though I haven't tested the patches for kernels != 5.15).
Bonus: the kernel + initramfs + firmware files get bundled into a boot.img RPI4 ramdisk image (making it easy to use for a secure boot / tryboot / failover configuration, though I haven't tried those yet).
Finally, I isolated the interceptor-specific scripts using a configuration flag, so if you have other projects using an RPI 4, you can re-use these scripts to build custom Debian-based root filesystems + kernels.
Now, I have one the last binary-only part remaining: the swconfig .deb file... if someone could tell me the steps (or give me the DEBIAN control / rules files) to build this, I would be grateful.