How cool! It's frustrating how tedious many PDF workflows still are. I've been building something similar in this space[0], but web-based where you visually specify the area to edit. The biggest issue for now is the cost per edit as the Pro version amounts to roughly $0.15/image. However, with some finessing, the original Nano Banana seems to do a great job as well. Have you explored UI-based approaches yourself by any chance?

[0]https://docusera.com/