What more do you want? I just ran a game the other day for a group of level 16 PCs and because of time constraints only had a handful of fights (I usually try to do 6-8 encounters between long rests). Because it was a turning-point in the campaign it was more difficult than usual. I had near TPKs, using a variety of monsters.
I'd like to see more of what you described. Mostly more of the tactics. My group is a 50/50 mix of people who enjoy tactics and those who want an arena style slug fest. I want ideas that let all of them have fun.
"Use better tactics" is partially helpful. Some of my players are at least equal to me as tacticians, a couple of better than me at using terrain. That's great because they challenge me - it is also a problem because there are six of them vs. one of me, so I'm always interested in seeing how other DMs use monsters to push players to their limits.
I have no trouble making modifying monsters to make them more powerful. Tinkering with stats is always a fun way to confuse the players who also DM their own games think they know the monsters. But it is still hard to surprise them. Last session I did that by having an Archmage use Arcane Eye to target spells based on sight. None of the players had considered that a possibility and everyone enjoyed the surprise even though it was in the form of a Meteor Swarm that scared the crap out of them.
Clever use of things like your orc warlords giving advantage is the sort of thing I'm looking for. I've used orc war chiefs before but I don't think I've used them at distance where they could avoid melee attacks while still using their war cry. Now that you've described it, it seems obvious. But it didn't occur to me so reading your tactics was helpful.
The generic how-to ideas (i.e., take advantage of cover and environment) are less interesting to me because I do that. But other DMs may not have thought of it so it is also good to include in this thread.
Some things are good ideas for many groups but not useful to me. For example, I have two players who are adamantly opposed to bringing flanking into the game. I realize I could invoke Rule Zero, but I don't like that sort of heavy handed play. I respect that one-third of my players are opposed to using certain optional rules even if I think they are being narrow-minded. If imposing the rule makes the game less fun for them that's enough for me.
Transformations. This one is a little trickier (and to do justice would take more time than I have) but instead of making monsters big bags of HP, have them change at certain points. At a 2/3 of their HP they lose some AC but start blindly flailing getting more attacks at a bonus. At 1/3 their HP their acidic/demonic blood is spraying everywhere causing automatic damage (or constitution save for half). When they die they explode.
This sounds interesting. I'd enjoy seeing more details of how you have used this when you have time to elaborate.