Google has done way more research on this topic than both you and I collectively and they settled in on 4 interviews being the sweet spot to get enough signal to be 86% confident, while not wasting any more of anyone’s time than needed chasing after single-point confidence improvements. In my experience, I agree with this. I’ve been through 6-round and 3-round (both to offer). Even as a candidate I guess I feel like i wanted that fourth round. Kinda hard to tell what a company culture is from just three meets. And after six rounds I was just freaking exhausted and didn’t really have a high opinion of that company-they couldn’t seem to figure out a clear mission/vision for their product and I thought their overly complicated and drawn-out interview process was a reflection of that.

Google goes into more depth as to why the three-tech + 1 behavioral/cultural model works for them. They call it a work-sample test.

The best predictor of how someone will perform in a job is a work sample test (29 percent). This entails giving candidates a sample piece of work, similar to that which they would do in the job, and assessing their performance at it.

Both articles linked are well worth the time to read. Hiring is a messy and inconvenient process for both companies and employees.