in my experience (historically), B1 is fairly stable, as Apple has had months to put it together. The wild ride portion of the beta cycle for me tends to be B2 or B3, as they start implementing tweaks, and tend to break things. By the time we get to B4, things settle in pretty well, for the most part.
I do not expect iOS 26 B2 until next week, but would be quite happy being surprised today for the new beta. Or, do we see something with iOS 18.6 today?