Longsac, I disagree. The reason you get much higher income from 12,000 compared to 6,000 is mainly because of the dockyard. You should only use farms if you can't reach the magical 12,000 or 24,000 number, and barely enough for that. In fact, you don't need them at all, since I don't see any cities that can't reach 12,000 without farms. Farms will get you there faster but it will cause more problems with public order in the long run. The move from 12,000 to 24,000 doesn't increase income much but increases squalor a lot.

For me, 24,000 is the magical number for my main troop producing cities and 12,000 elsewhere. In my Julii campaign, I built up the Ceres tree, got to 24,000 then demolished them and replaced them with Jupiter. My public order is so much better that I have less than 24,000 in many cities with the imperial palace.

Look at your garrison size. 4 peasant units can easily impose 80% (the max) public order in smaller cities. Once you get to 24,000, it drops all the way to 15-20%. Your squalor will be around 75% at that point. Having farms at that point will do nothing than increase squalor unhappiness by 5% per point and you'll run out of happiness upgrades soon. Moving to monthly games cost 400 per turn and that's quite expensive.