That seems like a pretty decent outline. I think I’d do only bulk/batch submissions to avoid having two methods to write, debug, test, and maintain.
Or maybe a single bulk/batched job could avoid the rate limiting issues and then have just one way of submitting stuff (just in a batch, never on a per-post level).
A version that submitted to a single endpoint might be $2000 for something that appeared to work and had minimal error handling to $5000 for something with at least some specs to do testing; and maybe could handle notifying multiple endpoints?