Interestingly, despite it being much more detailed and a lot more process and procedure than what I currently do - which is more akin to the version 0 described, but in parallel - we come up at the same final problem: reviews and quality assurance.
I sign off the code I merged, part of company policy but also just to be sure it is actually decent. But reviewing has become the real draining bottleneck: even stacked PRs, if that total 5-6k lines is not a 5min job. Even if I brainstormed and set the plan, that's really the part that doesn't scale right now for me in this. But the author is very shy about that: either the changes arent that big in the end or they trust the process enough to review in a more casual manner. Being equally untrusting I can't do that ...