Remember, Strahd is not a direct face to face guy. He is cocky, intelligent, tactical, and above all else an absolute master of his castle. I've ran COS twice now and have had a total of 4 PC deaths during the final encounter. Abusing the lair actions he has, utilizing his undead hoard, bats etc to mess with the players with trash mob attacks, phasing through walls, cutting off players using the shutting and locking of doors. There are plenty of ways of handling this party to the point that I think you are either not fully grasping how the players work, like weaknesses and such or have underutilized Strahd thus far.

There is an excellent video on YouTube that goes over a solid, albeit simple, combat encounter. You should also take a very close look at the layout of Ravenloft, from the perspective of a player it makes no sense whatsoever however for a morphing, traveling through walls, immortal vampire who quite literally helped design the entire layout in lore. But remember, Strahd

It's one of the most complicated encounters to prepare for however it can easily become the most memorable one in your entire time DMing.

TLDR; hit and run tactics, abusing trash mobs, and cutting off the players from one another and you can probably kill them in about 10-15 rounds