I haven't done that route myself, but it sounds like mostly scrambling with a short bit of technical climbing. This would be one to do because one wants to climb Long's, more than because one wants 1000 feet of 5.3. The fact that it's preceded by a fairly long hike in might also take it out of the running. I think the standard route of the First Flatiron is definitely a good route for the "1000 ft of 5.3" purpose, although the 1st pitch is generally called 5.6, more for the lack of gear than the difficulty of the climbing. The 3rd Flatiron is closed so many months of the year, I actually haven't got on it yet (closed Feb through Aug). The other thing going for the Flatirons is that they're near lots of other "harder" climbing that doesn't necessarily require a huge walk-in to get to it, or on the flip side, lots of cool backcountry (but still easy) routes that do require big hikes.
Also, I think 1000 ft of 5.3 is a perfectly noble goal. It's nice to climb when you can just go without worrying about scaring yourself to death, and can enjoy the scenery!