HM cannot add extra damage per slot level.this would only fly if HM added +1d6 damage per spell level upcast.
then you can add +5d6 per attack as 5th level spell. and +5d12 with your version of capstone.
also your "free" casting of HM are 2nd level at 11th ranger level and 3rd level at 17th ranger level.
then at 13th level you get +2d6 extra damage from "free" HM and that maybe enough to not wonder why are you not concentrating on some 4th or 3d level ranger spell at the time.
and as this "free" HM scales with Ranger levels, there is no danger of MC cheese dips.
Hunters Mark can be snagged by other classes via feats as it's a 1st level spell.
I already explained this.