Best farming stages are

    GA-8: Coagulating Gel
    GA-7: Loxic Kohl
    GA-6: Sugar pack

Farming efficiency: Kohl > Sugar > Gel

For new players: you can ignore furniture in event shop, you will get it year later for free when GA will be added to side story tab.
For veteran players: buy all furniture, Enforcer pots and you must complete at minimum these stages: GA-1, GA-2, GA-3, GA-5, GA-6, GA-7, GA-8, GA-EX-8 CM
Credits to Daniel O'Brien on Gamepress