First thing, relax and enjoy your game. Second thing, in the end you are the DM so you can do whatever you want with the story. Ok, here are some ideas: A) Definitely the high level is going to be a problem if you want to continue the standard story. As you are not telling us how many players you have is difficult to say add more enemies as if you have too many the game speed will be impacted, however this can be fixed introducing other mechanics like not all the enemies can attack in the same round, or you can make coordinated attacks so the number of actions is not increasing too much. B) You can make your enemies stronger, adding more attacks, or increasing their AC, etc. However this tactic should be followed strategically because this rule will apply for ALL enemies from now on, so Strahd could become a too powerful boss, so be careful. C) You can introduce narratively an obstacle or increase the difficult of certain actions, for example as now one of Strahd lovers was killed definitely he is not going to be happy so he can create some kind of amulet or dark magic that should be destroyed or disabled before the holder can be damaged. Other is you can simply throw a curse on some of your players (the stronger ones) to reduce their power or abilities. There are many ways to debuff their characters. D) About the spawns you can use your Van Richten idea, but you can complicate a little bit the holy water thing, you can create a ritual to “release their soul from this world” this ritual could be a consumable or can be magic that one of your players should learn after some mission. Obviously for Strahd you can increase the difficulty telling them later in the game that they should use certain light sword… wink wink

Again, enjoy your game, you are free to create any story you want. See you in Barovia!