It should be easy to replace all images in a pattern or set of patterns with images from the media library or a new set of uploads. Some of the following scenarios seem worth exploring:

• Open media; select a bunch of items; offer a new option next to "create gallery" that's "fill in selected pattern" or "pick from patterns" where you can browse pattern categories like "gallery".
• On pattern blocks (wp_block) offer an option in the toolbar for media assignment; it'd open the media library contextually, and allow you to select as many images as image or media blocks are in the current pattern.
• When uploading multiple images, make drop zones smarter by allowing to not just create a new gallery, but if dropping over a pattern with image blocks, fill them in.