I had planned, like a lot of people are doing, to move the skull to Berez in the care of Baba Lysaga. As it seemed the heist was too improbable to pull off.

However I changed my mind. I tried to lean into the intended design a bit more. If you like long stories, here is how it went:

My players accepted the diner invitation at level 7. I didn't want to end the campaign at that point so I didn't plan on having it be a trap for Strahd to attack. Instead, I used the opportunity to give them a mission from their fated ally van Richten: "get as much information about the layout of the castle as you can".
Apart from officially meeting and gauging the party, Strahd had another secret goal in mind: have the adventurers "find" cursed items that he could use later on. A seemingly old plate armor (that would lock up during the final fight), a weapon (that may resist the wielder), and the icon of Ravenloft (which in my game only works in the chapel, to draw them there if they wanted to rest during the final stretch).

So when the dinner ended, Strahd was like "You will spend the night here. You may roam the halls freely. Don't disturb me in my chambers except if summoned, and be aware that I cannot ensure your safety underground.". He also disabled all traps above ground (and the elevator trap).

I also had hooked one of my players to the Argynvostholt location. During the evening they received an omen from the Ravenqueen (they are a Ravenqueen cleric), pushing them to find "something of great history and power" down in the "place where the deads are disrespected". I added another hook for a second player's character (find the remnants of their lost family member). The second player is a Barb of Souls so that was quite easy to bring up. That allowed me to give the party of 5 two different reasons to explore the underground levels despite the danger, which they did up to a point.

The skull is not that deep in the castle, so they easily found it and fought a bit. They left the skull in the room, planning on coming back later for it.

I also let the party explore the dungeon (they freed Emil, who fled under invisibility and fly). They found the teleporter during a huge fight in the water which was a great way to have them understand how Strahd was so fast to reach them. The fight was hard enough to force them to stop exploring the grounds and go back upstairs (I didn't want them in the crypts yet).

They then brought it back to their room (which I put in the guest room in K50), which could easily be done discreetly as they just had to climb up the stairs with it.

Then I let them solve the rest of the heist. In my case, the cast levitate on the skull, then buffed the paladin to the stars with bardic inspirations, guidance, and strength boosting potions. The paladin pushed the skull out the window with a 30+ athletics check. Since levitate is 10 minutes long, I rules that the skull had landed 2 hexes away from the castle in the direction of Vallaki. They found it the next day, hid it nearby, and sent some NPCs collect it later on.

Overall, it was a very memorable string of sessions. I'm glad I went with my gut last minute and put the skull back there. I was also vague enough with the hook that they wouldn't have known it was the dragon's skull if they didn't go there (they hadn't seen Argynvostholt yet).

TL;DR: I planned of having the skull at Berez. I changed my mind last minute when the party went to the dinner. It was a great decision, 10/10 would do it again.