As another commenter mentioned, it's common practice to shove all your device's functionality into one characteristic to reduce service discovery time (and, in some cases, memory footprint in firmware). With that in mind, can OpenBLE be used to describe the structure of the bytes being read, written, and indicated by the characteristics?
OpenAPI is useful because it not only generates a frontend for testing an API, it also can be used to generate the skeleton of an API's implementation. Can I generate Zephyr macros using this tool? Can I generate Android or iOS code? A complete tool should be able to generate firmware, Android, and iOS code for both a GATT server and client.
As a professional firmware engineer working in IoT, I'd advise against hanging your hat on Web Bluetooth. Bluetooth is already rife with security problems, and exposing direct control of it to a browser is asking for zero-day nightmares. This is beside the antipattern involved in asking web apps to directly control a peripheral's communication interface.