I'd say returning 200 for all successes is reasonable if the responses are simple.

Returning 200 for an error makes no sense. Having 400s and 500s is the simplest way to have observability over protocol behavior (think logs, error rates, etc). If you use all 200s, you'd have to re-implement observability by yourself, so you lose simplicity that you gained by ignoring those statuses.

It's the same thing with caching stuff. You could implement those outside the protocol, but then you'd be writing your own protocol (trying to be smarter than decades of engineering efforts).