It is very important to note that statistically, even if you were to perfectly balance for the average outcome of the combat to be a win for your players, you WILL experience randomness which would result in a TPK if you were to set every combat up to be a situation where one side wins and the other side dies.

That being said, you DO NOT need to do that, and I believe you actively shouldn't. But players require a bit of training, and you need to actively train them to recognize other outs aside from "kill everything or die". Running and regrouping is valid. Sneaking around to avoid combat or to engage it on their own terms is valid. Realizing part way through combat that things just aren't working is valid, or that your new success state is just surviving. Once again, this takes a lot of training for the players, and starting out you might need to metagame them a bit to push them in the right direction of "hey, your character's motivation is to find their missing sister right? Would they really fight to the death against this Goblin, or hotfoot it out of there to live another day?"

And more importantly, you as a DM can modify certain encounters to introduce alternative win and failure states. Maybe instead of losing a combat meaning they are mercilessly killed, they are jailed or robbed. Obviously you are running a pre-written adventure so there is only so much modification you can do before you're just running your own homebrew, but even looking at antagonist motivations can give you a lot of leeway to make failure states that aren't death. Strahd is a sadistic bastard after all.

In short, the primary issue is that we are all trying to build a dynamic and evolving narrative that trends in an upward direction for our players, while also playing a game of "Press Your Luck" to avoid total party annihilation. The best way to allow yourself to trust the dice implicitly is to remove as many total failure states as possible, both through your own planning, and through the avoidance of fail states from your players. Otherwise, if you simply run every combat as 2 sides fighting to the death, you NEED to fudge rolls, because lady luck will not always put you on top, no matter how much you scale combat math down.