i dont think it is possible to get a prefect economy on a server for many reasons
1.as you said people can make the giant farms withing a few hours and then afk it to make lots of money
2.mob arena would defy the way of skyblock as it involves a lot of pve whereas in skyblock the goal is to just survive and get your island to be bigger and better
3.if you would put low prices for the sell signs then there would be a depression in the economy
thats all i got i just wanna say that if possible setting up a low level economy might help the players to start quick and efficient in the server
