I have a couple of potential solutions
1. You can only menhir to a kingdom that is at the very least 7 days old (3 days for 3x servers)
2. You will keep all resource fields, but you can only menhir if you have a 4-4-4-6 village, and you can only be menhired to other 4-4-4-6 abandoned valleys.
*1 makes it so that for sneak attacks with menhiring players next to an enemy treasury has a longer delay, requiring a king or a duke to settle a new village there before doing the normal activation process. this gives time both to shut down the village before it activates, as well as giving the usual time to react before a menhired player can attack you. You could even make it so that activating a treasury takes 24 hours instead of 12 hours, activating and deactivating treasuries is not really something that should be done on a whim, but should be a large decision made by the kingdom.
*2 This will stop the resource pile boosting, and will also not punish newer players when they menhir after a couple of days of building resource fields. Having to rebuild your resource fields after moving can be a bit of a daunting task because it feels to them like they just wasted the last couple of days of upgrading resources.