Yeah well, at least it seemed promising...

I've played a few turns more, and have seen three more guilds, in paris(french), dijon and marseille(both milanese), and they've all been thieves guilds...

As of now, it seems like it may take a bit more to make it balanced. But there's impossible to draw any conclusion, I haven't seen more than 6 enemy cities( of which 4 had the thieves guild, the other two had none, but were both small/early).

However, one thing that might make matters brighter, is that when Algiers and Zaragoza got stone walls, Zaragoza got offered merchants guild, which I accepted, while Algiers got offered Theologians guild first, then thieves and then finally merchants guild that I accepted.

None of those cities had produced any spies, neither by me or the AI.

However, if thieves guild still is the dominant guild, I might have another solution:

We all know that the AI uses a lot of spies, and as we kill a lot of them too, they must be producing them rapidly, and that means that they cannot be centralising their production. So, they're producing a lot of them in every province. As the player, you produce them in a select few provinces, so capping the global triggers stops you getting offered thieves guild in every province. This might not work on the AI, as they produce from every province, thus getting enough points from settlements specific triggers. If the global change doesn't work, I'll try halving the trigger on training spies(from 10 to 5).