C) Limiting the max number of units in game and thus reducing the A.I incapabilities. Instead of "more", we (and the A.I) would be looking for "better" units. For this the game should be fixed and balanced accordingly (defense building to strong, units not upgrading, etc.)
I think this would be the best solution for now (simplest).
Limit the number of physical units on the grid, make unit quality and individual unit powers deeper.
Carpets of doom are horrible because
a) AI can't manage them
b) They are very annoying, tedious slog for the player (infinite traffic and logistical problems, unit swapping, etc)
c) They devaluate individual units experience
Increase unit build times and maintenance so there are less of them on the map (like 2-3x less). Balance city defenses and other costs so it's enough for conquest.
Make individual units more valuable with extending promotion system, experience and maybe inventing entirely few new, small minigames altogether (such as training, discipline, morale etc)
I'm pretty convinced AI would be far more capable of managing few more "unit quality" rules than gigantic mess of dozens of units swarming entire continents. It would also make warfare more pleasant for the human player.
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Otherwise, if devs had more time and resources, I would be the great fan of introducing limited stacking - either via "hard caps" or soft gameplay limitations (such as aforementioned ideas of logistical limits).
I greatly despise unlimited stacking in strategy games and 1UPT is "less bad" but... notice how I said "less bad", not "better".
Unlimited stacking = quantity always wins over quality, no depth in warfare, boredom.
Unlimited (many units) 1UPT = incomprehensible mess for AI and tedious pain for human player.
Better solution lays somewhere between those two extremes.