I have no experience with dehydrating horseradish, so can't speak to that.

But . . . How long are you waiting before packing in vinegar? The degree of heat increases exponentially depending upon how long you wait. Add vinegar 1 minute after grating and the horseradish is mild. After 3 minutes it's much much hotter. The gain after that is much less, but you could wait 6-8 minutes before adding vinegar for the really really hot stuff.

Also, horseradish root once dug will keep 6-8 months in the fridge, which almost gets you to the next season, so you could grate some fresh.

Carol