There seems to be two camps: 1) traditional, modern or vintage steel framed bikes, and 2) carbon fiber or titanium bikes. I have been surprised at the number of riders using carbon frames on 600Ks or more.
There are variety of factors, beyond frame material that contribute to a bicycle's ride qualities. Knowing this, you should be careful not to make too many assumptions about a particular bicycle's frame material and realize that a bicycle's frame-geometry and your choice of tires and fit setting will affect your performance as well.
Richard Cranium would not consider frame material as important as frame geometry. Tire type, size and pressure have as much to do with performance as frame materials.
Without specific information about how you intend to outfit a brevet-bike that's all that can be said.
I am a generally lightweight, small-frame person..It seems to me that a lightweight bike would be the best option for me, but I am concerned about comfort over a 600K or more.
See answer above, again tire-size and the gear you carry "on-bike" are big factors in your overall brevet comfort.
Would I be discouraged & slowed by the extra weight of a steel bike? I barely finish some 200K's within the allotted time as it is.
See first answer. Tires, and wheels - and of course on-bike gear are bigger issues than frame material.
Your concerns may appear reasonable, but
your attention to frame material as a large part of brevet successes or overall cycling performance represents an immature perspective toward Randonneuring. You'll see what I mean in ten or twenty years.