Ok, I came up with a possible solution for multis just now so it may have many flaws that I have not thought through. Or perhaps this has already been suggested, anyways I just wanted to quickly note this down before I forget.
To prevent multi farming within the same kingdom, kingdom members should receive absolutely zero resources when attacking kingdom members. I know there is already a limit of how much resources you can get from attacking own kingdom members, but this solution would absolutely discourage any multis joining the same kingdom as multi farms. You might be thinking that this solution wouldn't stop multi farming because multis could just be farms without being in the kingdom, you are absolutely right. Instead, this small change would help us better identify possible multis.
With the restriction in place, multis would be forced to be kingdomless members. Assuming these multis don't build troops, they would eventually be public farms available for everyone which helps level the playing field. However, it is possible for multi abusers to spike their multis and make it their own private farm. So I suggest another solution, like travian legends, players are not allowed to reinforce kingdomless members. This again levels the playing field allowing everyone to farm these multis. Only problem here is that abusers may start building defense in multis to prevent their multis from getting farmed and dodging attacks to only let their troops pass to reap the rewards of their multis. Unfortunately, I was unable to think of a solution to this problem, but I believe it makes it dramatically more obvious in determining whether players are abusing multis when a certain account is only dodging attacks from certain players. You could argue that abusers may ever so often purposefully kill their own troops or let a few other players farm, but I believe this would drastically hinder multi abusers' efforts in abusing the system and possibly stop abusing it all together.
You could argue that multis could just be in a different kingdom and continue to be abused, but the same would apply with the previous point. Say abusers reinforce these multis from a different kingdom, or the multis build their own defense, it would again make it obvious what these abusers are doing when they only let their own troops through.
Although this may not be a direct solution to stopping multi farming, but I believe this helps create a checklist for moderators in what to look for by better defining the multi abuse, generate more evidence to these types of abuse and also hinder and discourage the abuse.
Edit:
-by not allowing multis being in the same kingdom, there should be no reason for a player to be benefiting from another player without any relationship, so this makes it more clear when identifying multis
