I would go to Colorado personally
There is no place in the USA where it is fall and light winter all year round. However, I find wheneeever I go to Las Vegas or Phoenix, my nasal passages dry right up. And here in Orange County CA it is good though not quite as good.. I find that dryness is a big factor with allergies and the desert or the LA/San Diego areas are pretty good for people with allergies.Therefore, I say to consider Phoenix or the like.
other than the south west not many places with out pollen pine and oak is high maybe the north west plains pollen would be short lived and the wind would blow it out quickly. sorry good luck
The Alaskan panhandle, Juneau, Ketchican etc.