Well, it can either be about WWs and FoY, or it can be about fighting as far as possible in GE/GBG. Or both. I wouldn't worry about an Arc in a Diamond farm except for Treasury goods. WWs and FoY will provide goods, coins, supplies, medals and FPs. I don't think they take any population, so that's not an issue. I would say that to maximize the Diamond farm, the major GBs to have would be the Alcatraz, Zeus, CoA, CdM, and the higher level military GBs, if you can manage them. To be effective in GE/GBG without taking up space with barracks, have one Rogue Hideout and rely on GE to provide unattached current era troops. The only concession to normal play would be some production buildings for event/DC quests in order to be able to earn more WWs. At least, that's my take on a Diamond farm. Currently, all my worlds are basically Diamond Farms, but they weren't always that way, so they only resemble what I've described here in an incomplete manner.