Dragon fight and strong party problems

I’m DMing Dragon of Icespire Peak for four friends from my university. This is their first-ever RPG experience. They’ve completed every quest in the book and are now one session away from the final dragon fight. They just defeated a buffed version of Gorthok.

Party Composition

• Vengeance Paladin (Sword & Board)
• Samurai Fighter (Archer)
• Arcane Trickster Rogue
• Great Old One Warlock
• All characters were created with point buy.
• Each character has a +1 weapon and is reasonably optimized.
• They are all level 6.

I’m struggling to create balanced combat encounters. Their damage output — especially the Rogue and the Fighter — is extremely high, and they have a lot of resources.
As an example, I ran a “dream” encounter where Savras gave them a trial fight against a young white dragon at the Shrine of Savras. They obliterated it in just two rounds (I capped the fight at 2 rounds because it was only a vision anyway).
I want the final fight with Cryovain to feel fun but challenging. Since they have a scroll of Revivify, I’m willing to push the danger a bit harder to make the encounter memorable without feeling unfair. I imagine a series of encounters instead of just one so I can get some of their resources.
I plan on using the Dragon Rake from this post https://www.reddit.com/r/UnearthedArcana/comments/197gnjp/complete_white_dragons_9_variants_with_tactics/
but I need help with planning.

Any tips?