I have 8 farms, all IA fighters. The end point of every diamond farm is to auto battle 64/64 imo. Space should be prioritized to getting to that point, not placing more WWs. WWs fill up all the space not dedicated to fighting. This is of course from the perspective of absolute minimum time for maximum diamonds, while being completely self sustaining.
The problem I have with a negotiating farm is it requires management of your friends list and a lot of trading. Negotiation takes a whole lot longer than auto battle as well, and it has the potential to fail at higher levels.
That being said it takes a whole lot more work to get a diamond farm to battle proficiency. Additionally, even at 10/10/10/10 zeus, coa, cdm, and traz I still go in on 3 or 4 battles to manually fight. Right now I'm not really sure what to do to get to fully auto battle. I'm very hesitant to go the arc route and stratosphere my attack GBs....way too much effort.