You've actually almost exactly described how PiBakery works - except that PiBakery obviously can write the image as well. Once the image is written, PiBakery just writes some scripts into the /boot/PiBakery/ directory, which is checked by a systemd startup script, and executed.

And if you want to create custom blocks to add in to PiBakery for your own tools/scripts, you're welcome to do that! I have a guide on the PiBakery website - www.pibakery.org/docs/contribute.html - that describes this process that you can look at if you want.