Endurance, Touring and Gravel are mostly just marketing labels and there is a high degree of overlap. From the description of your rides, it sounds like a "touring" bike is what you want, but as soon as you put fatter tires on a touring bike you get to call it a "gravel" bike.
You mentioned comfort over 50+ miles, and to me that says steel. With your budget you can afford a very nice chromoly touring bike. Look at the Surly LHT and also the Crosscheck. You'll get lots of other good recommendations but those are great starting points.