That’s probably a good summary Mike – there’s a page I wrote here http://www.penninemegagames.co.uk/a-guide-to-megagaming.html
I gave up trying to write a full definition as different people value different elements. They started as operational wargames under Paddy Griffith and Andy Callan, but the political side of most games is very popular. In my Justinian-era game, there was a resource management element on a board for governors, a simple campaign system on open maps for the generals (allocating unit cards to three zones, plus combat cards and reserves if necesarry) which resolved battles in 5 minutes or so and a political support/impressively blinged buildings game in Constantinople, all the while Justinian was trying (and failing) to manage things and the Sassanids made deals with people. Standard wargames rules take too long, but even in a military game you want (ideally) at least three layers of command to build in friction and fog of war.