Thanks for posting. Looks very nice!
Just for comparison, here is a gemini-created code that I modified to allow custom coloring - no attempt at a background image yet.
I had Gemini utilize the variables for Amazon KDP covers, here. (Also not as nicely separated into template.typ & real-file.typ as yours.)
The one addition it has is the placement of a barcode, which I included as a “barcode.png” placeholder image. Steal whatever code you need from it!
template_KDP_Cover.typ (rev 0a) (6.3 KB)
barcode.png
I have yet to manually verify that this layout actually conforms to the Amazon KDP book cover requirements…use with caution!