Making sure I am on the same page as you are...you are saying that there was a major viking raid in England approximately every 3 years. Because you consider the base D&D world to be "more deadly" than the real world circa 800ce, we are going to double the frequency to a major raid every 1.5 years.
I am going to attach a map of ALL the evidence of vikings in Wales (found in internet, cannot claim the source is 100% accurate). I am going to include a second map I put in MS Paint for 5 minutes to add in some D&D words. I'm going to attempt to show you the extent of "raids" into an area, and how a small population of little folk could exist fairly safe tucked away from the action.
View attachment 131893
View attachment 131896
View attachment 131895