However, once the PCs got into the castle, they made a beeline for the crypts, which they spent a whole session systematically looting one by one. When Cyrus Belview came down to say the ceremony was starting, the PCs gleefully shoved him into the tomb reserved for Ireena, shut the door, and went back to looting.
Strahd had no issues with this at all?
He's just...
letting them ransack his castle, and murder his minions?
You even have a Doom clock set up man (the Wedding!). Let the PCs know that if the Wedding is completed, Strahd will complete his Apotheosis, and become some kind of murder-machine, and they'll be trapped in Barovia for good.
Once that's over and done with, have his minions attack them. Constantly. If they nova (and then rest) have a Vampire Spellcaster (Vampire Spawn with Mage spellcasting tacked on) plus Mooks turn up and Dispel the LTH, and hit them again.
Have them find out about (the sword, or the amulet of ravenkind, or the Heart). Steer them there with another Doom clock (find the artefact/ destroy the heart by time X or else bad thing Y happens). Hit them with repeat encounters as they go. Keep the pressure on them.
Increase the Heart's effects to grant Strahd advantage on his attacks, disadvantage on all attacks v him, Resistance to Radiant damage not caused by the Brilliant longsword, double the HP, and Max damage on all attacks. Make tackling him head on (without destroying the Heart) a suicide mission.
When the final showdown happens, have Strahd appear wearing his magical +2 Breastplate, Ring of Protection +1 (20), and Flaming longsword (deals Necrotic damage instead of Fire) and a plethora of Vampire mooks. Ensure you use his ability to walk through walls to its maximum effect. Change his spells (he is a wizard after all) to tailor to the PCs weakness (Int saves Im looking at you).
There is SO much you can do, that you arent man.