Yes, building WW Diamond farms demands patience. The nice thing about Iron Age and EMA Diamond farms is that you don't have to spend 5 Diamonds to collect them quickly. With 9 cities it is too much trouble to collect my TE and above cities (4 of them) manually, so I use the auto collect most days on them. My current strategy is to prioritize WWs/FsoY and buildings that have attack and/or defense boosts for my attacking army. I don't worry about FP, because that's beside the point to a Diamond farm, and so are goods. DCs/GE/events can be managed quite easily without specifically targeting goods or FP production in your city. In the interests of time I rarely play into the 2nd level of GE in my lower era cities. I do occasionally get Diamond rewards even on that level, though. The only city where I get into the 4th level of GE is one of my FE cities. FE is my favorite age because I can auto battle all the way through the first half of level 4, even with an unimpressive a/d boost of 432/237.
Now you may wonder how much return I get on these WWs with such a laissez-faire attitude. Well, I used to buy Diamonds when they had special deals. At one point a few weeks ago, I spent down from a total of around 48k Diamonds to around 36k. Right now I am standing at almost 41k Diamonds without spending a dime. And yesterday I spent 1200 on Diamond expansions in a city that advanced to LMA. I also am not shy about spending the occasional 10 Diamonds to finish a negotiation. Not to mention reviving dead artillery in my younger worlds that don't have a Traz.
As far as space and the availability of WWs/FsoY, I have several of them in my higher era cities sitting in inventory right now. I might put some down when I sell most of my production buildings after this event. Speaking of the event, even though these cities are mainly set up as Diamond farms, I will complete the entire event in all 9, getting a fully upgraded main building in 6 of them without spending Diamonds. In the other 3 I did not get far enough into the previous event to get the Event Surprise Box, so I'll have to spend a few Diamonds to get that last upgrade in them. Less than 5k Diamonds. I'm still deciding whether it's worth it in those cities or not.
Bottom line is that with patience and discipline you can build Diamond farms that provide you a steady income of Diamonds. Definitely enough to spend fairly freely on one main world.