IF your players are new to D&D 5e, and especially if they are new to D&D AT ALL, I would run an alternate-dimension starting adventure, whether home-brewed or something you find online, to take them through from 1st to 3rd level, and a little bit of playtime at 3rd level.
Then, when you're ready to take them "Into the Mists," you can let them take those same characters, but "from an alternate dimension," so that they are not exactly the same.
Hear me out. Learning D&D 5e takes a bit of a learning curve. Those first few levels are the ideal time to learn the rules and how everything works, while dealing with low-stress obstacles. Let them learn HOW to play, HOW to use their spells, HOW to use their abilities.
I saw a cool story on one of those Reddit Read videos, about a DM whose players did nothing but hack and slash, so he set up a puzzle room, where they were locked in and the only way to open the path to the exit was to lift one panel for each PC. Each panel had a symbol on it to correspond to the PC. So, the Bard had a harp, the Fighter had a Sword, the Cleric had a holy symbol, that sort of thing. There were 4 skeletons that shouted "Freedom Takes All You Have!" before attacking the party. After the last skeleton fell, the DM started a count, and a minute later, they rose again to fight some more.
It was an easy enough battle, but since it kept happening over and over, they would eventually be worn down, if they didn't get that door open.
They started using some abilities, such as Cure Wounds or Second Wind. When the first PC had used THREE of his abilities, other than pure hack and slash, his panel opened. Then, the other PCs realized that they had to think outside the box, and use some of their abilities, too. Eventually, everyone had used at least THREE of their race/class abilities. They explored their options. They used items to help them. The bard gave Bardic Inspiration to his team-mates. They figured out how those abilities WORKED.
I used this in my own game, with a reward at the end, and it was cool, because due to a deadline, we had to stop in the middle of the battle, so they spent a week or two debating over how to solve this puzzle, and they strategized together, out of character, and really got to know their PC's abilities.
Add in a trapped hallway, and another puzzle room where the puzzle was simply "say the name of the Goddess whose face is on the decoration in the room," to open the door. Just to encourage them to talk, in character, and role-play a bit.
After that, they were in a "Sanctuary," where they were magically sealed until morning, with lots of fun stuff to do, and role-play time, where they learned about short rests and long rests, and just chatting in character for role play, and how to do card games or dice games or playing darts, or whatnot. The next morning, they left the Sanctuary with some prizes from the God, and went on their way, refreshed and ready to start the new day, and with some experience and some better knowledge of their characters.
Anyway, you can come up with whatever you want, but I encourage you to give them both time to learn how their characters work, and to learn who their characters are, and also to play-test a few of those early choices. They may want to CHANGE things, when they get to level 3.
Then, you take them to the "alternate dimension," where they are not quite the same. Let them decide if they want to change their characters up a bit. Maybe they love them as is, but it's also possible the bard may regret their spell choices, and unlike the sorcerer, they can't swap spells at level up. But this time, they can, and they have a better feel for how they mean to play the character, and what works best FOR THEM.
Then, you take them "Into the Mists," and let them go with their "new and improved" characters at level 3, and a full (or at least more extensive) knowledge of how to play the game, and what they want from it.
That said, I would definitely use the home-brew part of this beginning adventure to foreshadow some stuff to come. Such as encouraging them to have a cleric in the group. Throw some undead at them. Throw some lore at them. use your NPCs to give them a heads-up, in a roundabout way. Maybe tell some "ghost stories" around the campfire, just to get a bit of that scary vibe going, just to see how the players react.
Seriously, if you have horror-haters in your group, don't spring COS on them, without some warning. They may not want to go there. Some people don't like gothic, and that's fair.
Anyway, have fun! Use the beginning adventure to teach your players, and teach yourself, and foreshadow some stuff, and give them a chance to correct their beginners' mistakes before you toss them into Curse of Strahd. It's very sandboxy, and it's very challenging, as well. Shopping is a challenge, with the stores selling only items up to 25gp in value, but charging double the prices, so letting them start with some gear already is good, too.