Questing should not be the primary end game quality grind, and especially should not be infinite as it is. All of the arguments against infinite anvil spiraling apply to questing, except questing doesn't promote trade and (drastically) fewer people enjoy questing than mining.
I assume you've made questing this way is because you believe nodes can't be walled/monopolized by groups, or that the quality bonuses possible are not game breaking. I assure you it's trivial to prevent others from accessing your quested nodes & the potential quality differences are big enough to be insurmountable in combat/trade/stat grinding, IE the entire game for most people.
Some solutions in order of my personal preference:
1) Remove questing from the game entirely OR
2) Remove +quality bonus from quests OR
3) Cap the maximum quality of each node at the highest natural quality for that node type OR
4) same as 5), except only apply the +quality to the central continent OR
5) Make +quality from quests apply globally at the end of each haven year like a mini-reset akin to new minehole levels