Thanks for writing this out and sharing. I often find myself faced with similar issues, and making challenges appropriately difficult.
I’m sure that PbtA “traditionalists” would say that the difficulty should follow the fiction. While a fighter could simply walk up and slay a lowly minion with a simple Hack and Slash roll, they would suggest that a Boss Encounter requires overcoming various obstacles just to get close enough to threaten the enemy. There’s also the obligatory reference to the 16HP Dragon example.
My solution has been to leave the numbers as they are and adjust the outcomes based on the situation. Sometimes a 10+ is a full success and sometimes it grants an extra boon; sometimes a 7-9 is a full success or a success with a complication.
Do you have any thoughts on how to define “epic challenges” etc?