But there's a LOT of light in the summer, so observing Northern Lights will be more difficult then.

If it were me, I'd time the northernmost part of the tour to either end of May / very early June; or late August. You'll miss the midnight sun season (technically, in the latitudes you're planning to visit, the sun does set every day even if it's for a brief moment only), but you'll also have a lot less mosquitoes to deal with.

--J