Glenn
"The Tarptent Rainbow was actually my first purchase, but it heated up too much'
Can't remember what tent I was seam sealing the other day in the sun (possibly a Double Rainbow...) but it definitely was warmer inside than out.
For some reason I don't think that I have tested the "liner" for sun protection but I know that the white fabric works for that in the Scarps .
I must test this soon, before summer is over...
Anyway Glenn, as a tip I would suggest you don't buy a Cuben shelter then...

ETSU Pride
Every TT shelter sets up dry. That is, you always set up the "rain fly" by itself or with the inner attached as it is the case with the Notch.
That is the advantage for example of the "duo mid" Strato Spire 1 and 2 over the other mids/duomids.
You set the fly and inner up at the same time (and no pole in the middle...)