I've been thinking about a workflow where PRs just get automerged into a staging environment of some kind, and then tooling helps ensure that all code, ALL the code in the entire project, is regularly reviewed together.

When lines change, they are put up for review. When lines haven't been reviewed for awhile, they are put up for review. Lines are ranked based upon the complexity of the context they are in, and complex areas of the code are reviewed more often.

Basically, stop focusing on individual PRs and instead focus on continuous review of the entire system with tooling that helps identify those parts of the code that are most in need of review.

Reviewing PRs is wasteful because the optimal way to review a system does not match the way a system naturally grows through individual commits.