I tend to stick to the same subject type, aircraft, although I do mix it up from WWII to Korea to Vietnam to WWI to between the wars back to WWII ... I have just recently done a true mix-it-up by building a couple of Bandai Star Wars figures, and have done some cars and tanks over the years.
The grandiose builds like you mention have been a huge problem for my modeling interest. In the same general timeframe, I did a 350th Lexington complete with all the photo-etch and fully loaded with its complement of Wildcats, Devastators and Dauntlesses. That one was a real time-consumming build that sapped a lot of desire from me to build. Even knowing that I decided that my next build (there might have been a small, quick build snuck in between, I don't recall for sure) would be Airfix's 24th scale Mosquito.
That one took even more time than the Lexington did, and that was complete out of the box. Yikes. After that I decided that I should stick with smaller builds, but I think I was away from my bench for longer than is typical, which would generally be no more than a week or two. When I finally got back to the bench, I picked out a series of short-run kits that I had - a couple of Classic Airframes molds and a couple of Special Hobby kits. These had lots of resin to work with, which normally doesn't cause me any concern but in these cases it seemed that those kits fought me start-to-finish.
The final problem I noticed was tackling the WWI subjects. I went through 2 different Sopwith Camel kits, not finishing either, before finally succeeding with a 32-scale Albatros.
I really have not had the same fervor for modeling as I did before I started tackling those large, complicated builds. I am still building but I am doing so at a much slower pace. I recently completed 4 Airacobras (3 in 48th and 1 in 32nd) that took me roughly 8 months to complete; normally I would have completed those in no more than half that time.
I am not totally done with doing the grandiose builds - I still have several kits that would qualify for that, a Fine Molds 72nd scale Millennium Falcon, 350th USS Texas, 24th Airfix Typhoon, among others. But I have not even begun to think of building anything like those. I really just want to build with a turn-around time that is not taking the majority of a year!