OK, another recent convert here
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Two observations about city and race relations:
1) Level 2 walls can keep a hostile city stable. Not always, but at least most of the times. Sometimes it's better to spend 2 or 4 turns and some gold to upgrade walls than to spend 3 to 5 turns of no income to migrate.
2) If there is only one enemy faction left on the map, your relation with that race deteriorates automatically and pretty quickly (faster than Tranquility's effect). There's no way to stay on good terms, you run out of upgradable/fortifiable cities soon. Just finish the map quickly.