I'm going to have to retract my defense that the late game doesn't get stale. Turns out when I said that I had a bunch of majors on the edge of promotion but no colonels. Now I've almost finished off the lategame upgrades, all my majors promoted and the game just started throwing new colonel recruits into my lap.
To keep things fresh I've tried out legendary before finishing my other caimpaign. One of my eternal complaints is how luck based the legendary earlygame is. Well, they minorly addressed that; you can now start out with a hero unit in your squad for gatecrasher. Templars unfortunately not viable (or at least no more viable than 4 rookies). Haven't tried reapers, but with a skirmisher I beat the mission on my first try. Was very careful to pick my first fight well, but there was definitely some malign force moving enemy squads so I could never quite ambush one at a time... in the end I got bored and took an opportunity.
Was it a good idea? Probably not, but it was fucking satisfying. Ended up with two deaths, one to a psi zombie and one to a mind controlled skirmisher. Someone on the internet pointed out that advent troopers have no crit chance and max 3 damage, once I knew that it was a lot easier planning things to avoid deaths. Unfortunately both the sectopod and officer really can't be allowed to take a turn ever, and your only really reliable kill on either is justice which has a 4 turn cooldown, so... still pretty luck based.