Tying off loose ends before entering the mists. (LMP into COS)

So I’m a relatively new DM who has just ran lost mines with my party of 3 (Rogue, Warlock, Paladin) and I’m getting excited but also nervous to take them into the mists once they finish up wave echo cave in our next session. I’m going to have them all stay at level 4 to hopefully not need any sort of rebalancing and naturally skipping death house. Wondering if you guys have any tips for the transition? I’m also introducing a fourth to the part who I think wants to play some sort of gunslinging Ranger build. Think I’ll play a bit into the whole plane jumping of it all and have his character be from a setting other than forgotten realms but I’m not sure how to introduce him. I havnt done much within lost mines to setup curse of strahd besides on of the pcs having their backstory tied to vampires destroying their home and killing their family. I had them find one of the rock seeker brothers dead at the entrance of wave echo cave with his blood drained and bite marks in his neck but that’s about it.

Im getting a little nervous about their characters backstories not getting a proper sendoff before they are whisked away to another dimensions where their life in the forgotten realms will have little to no impact. Trying to think of ways to tie it up nicely in wave echo or atleast give them a nice beat. Here’s what the characters have gotten so far in terms of personal development outside of the regular lost mines story.

The Paladin I feel like is the easiest. His whole thing has been looking for the vampires that destroyed his home and since I’ve made it that they are not very common he’s not had much luck. His god Najim has been very vaguely guiding him on his journey assuring him that he is on the right path but I feel like I’ll have the easiest time giving him some closure in Barovia itself.

The warlock is a hex blade noble whose mother made some deal with the family maid (actually a hag) and bargained a portion of his soul away. He’s got some sort of fiendish non discript fire patron who has given him a magical weapon I’ve called ember stone that isn’t all that powerful but has “an older brother” that may still be within wave echo that his patron has told him to find. Last session ended with him placing ember stone within the forge of spells and him being engulfed in the flame presumably to have a meeting with this patron. He hasn’t found this older brother and I’m thinking of maybe playing it to end up being the sun sword but still not sure how to play this final encounter with his patron before he goes into the mist.

The one I have the least ideas for is the rogue. He is a soul knife rouge and hasn’t really had much investment/ideas for his own personal backstory. The group has been playing lost mines pretty much as a mercenary group and I’m having trouble getting them to explore their group dynamics and relationships with one another more. So far his story has been that he was once a part of some crime guild that kept dwindling in numbers until it was just him. Ive sort of implied that he sold out his last few allies and has people who do know his past even encountering an assassin he killed before being able to interrogate. His conflicts are just all so material to the forgotten realms so I feel like I really gotta give him a proper sendoff before all that doesn’t matter. This will definitely be a good opportunity for him to truly “leave his past behind him” but still just saying like “well whoever was looking for you is sure gonna have a hard time now” doesn’t feel super satisfying.

Just looking for some ideas on incorporating this fourth player and how to tie off these original three in wave echo cave before the mists take them to barovia. Any tips for a newer DM taking on curse of strahd is also much appreciated.