Here’s what struck me upon reading your well-documented account.
You almost always have a wider min-max range on stage 1 than on stage 2.
I am three years older than you and my personal experience has been that when I do controlled tests, I need to do a very long warm-up if I want stable HR readings. Typically that means 25 minutes. I’d be curious to hear what Scott Johnston thinks of this.
That said, all the UA professionals had made clear that it makes no sense to worry about 4.41% vs 5%. If you were a little over 5%, you’d want to go down. But 4.41 is as good as 5.