For my part, I've never been much of a fan of Denmark's kit, and I don't play them much.
The bonuses vs cities always struck me as odd because it splits Denmark's focus between its pillaging mechanics and hitting cities. The only siege I can think of involving Vikings was the Siege of Paris, which was a bit of a disaster for the Vikings. It's pretty weird that their civ incarnation stresses attacking major cities as much as it does.
If it were up to me, I would drop the damage reduction and

Gold on city attack as well. Make the Viking/Longship promotions just the +25%

on pillaged tiles, and no movement to pillage. I would double down on the pillaging mechanic though.
UA:
Viking Fury or
Great Heathen Army
All Melee land units receive the Viking Promotion. All Naval units receive the Longship promotion. Embarked units have +1

Movement and pay just 1

movement point to move from sea to land. +100%

Gold from Pillaging and 50% of pillaged

Gold is converted to

Culture.
UU:
Berserker
No changes
UB:
Runestone
15

/

whenever you pillage a

GPTI or Village, scaling with Era.
So... we don't have secluded monasteries and enclaves in VP, but we do have academies and undefended Towns. The Vikings retain all their mobility advantages, but get no bonuses fighting well-defended cities. Putting the bulk of the pillaging bonuses in the UA makes it easier to control the city number snowball, which seemed to be the biggest problem. Hitting a critical mass of 10+ cities will no longer mean endless gold/culture.