My recollection is the US figured out pretty soon that the U2 could be
detected, but the claim was it flew so high it couldn't be hit. There
was photo evidence of MIGS from the overflights.

Yes, the way I remember it (I'd have to dig around to find the source)
is that initial expectations were that the U2 wouldn't be detected.
But on the very first overflight, US SIGINT sites saw the Soviet air
defense system lighting up along the flight path. And, as you say, the
U2 itself took pictures of MiGs trying to intercept it.