If you want to boot Debian on there you can actually build a complete Debian system from the package archives using either Debootstrap or Multistrap. Debootstrap is useful for building a Debian system inside of another Debian system, while Multistrap is useful for building a root filesystem to copy to another computer. If you can boot an initramfs then you can boot the root filesystem using pivot-root.

The Linux kernel can execute a shell script as PID1 from the initramfs, meaning that you can just execute the minimal commands needed to mount the main root filesystem and then use pivot-root to change from the linuxrc script in your initramfs to the main init process supplied in your main root filesystem.

You might have to get really creative with compression and static linking as the linker/loader clocks in at a few-hundred kilobytes, plus the gcc support library and the glibc for another few megabytes. Alternatively you could find a way to make the system calls directly and bypass the libc. Busybox will also fit on that disk and it ships with a mount command too, so you could get up and running with that pretty easily.