I had forgotten the spy effect. One of my favorite tactics since STW was using spies to cause rebellions. In RTW, I tried using this tactic, as well, but finally gave up. I could have some success against non-Roman factions but only minimal against the Romans.

I don't know how the AI sets taxes when it controls a city, especially when there's a governor, but I could have 3-5 spies in a Roman city, see public loyalty drop to 0, and still not get a rebellion. Often the AI counters by giving the governor "law" ancillaries, or law-giving traits. It was all too frustrating so I gave up that aspect of the game.

The more recent your city conquest the more susceptible that city is to unrest caused by spies. That's one reason why I like to recruit a spy within the first or second turn after I take a city.
I do the same, and preferably with a skilled spy, as "spying talent" often won't cut it. I also usually have an experienced assassin along to kill off enemy spies and assassins.