With human reviewers, I find that by the time someone has churned out enough of a solution to post a PR, they are already quite invested in specifics of the solution, and it makes it emotionally costly (to both author and reviewer) when someone says "hey, I'm not a fan of this whole approach, lets start over and do it this other way"
The emotional toll there is real, but this is exactly the moment when you expose the knowledge of that external dependency to the unbiased party that is the reviewer.
I like combining approvals to satisfy the urge for completion and closure, with a request for fast-follow refactor to better match the newly discovered model of interaction. (The worst code review experience I have seen is when a reviewer accepts it as-is and does a fast follow refactor themselves, depriving the author of the opportunity to learn and remain an expert in that area)
If you have these parties review each other's code, I agree that rarely brings much value.
I think the best way to understand our experience with reviews is to stop and say: in a few sentences, what do you expect out of a quality code review? (sounds like nothing in your case, but I am curious)
For bigger changes, of course you need feedback on designs. But that could easily be in the form of draft PRs.
I definitely would push back on anything that required feedback before PRs. That's way too much process. Just going to slow you down for no benefit.