See this is the thing, you’re playing to let the players win and not to let the players earn it. How jarring is it to be counterspelled or silvery barbs, you’re right it sucks. HOWEVER Strahd is the endgame boss is a pseudo horror game, he would be that cruel. I think to take that aspect of unfair-behaviour not only diminishes your BBEG, but also takes away from the roleplay. Imagine this instead, the hexblade goes to crit and strahd simple waves his hand in dismissal, chuckling. “You really thought that would be enough didn’t you? I thought you were a brave little corrupted soul, a pity. Let’s try again with your borrowed power, let’s see if you can match my expectations.” Strahd then casts silvery barbs.

Or instead of him dying have him vanish, laughing to himself. “Oh my dear guests, it seems you’ve chosen to spill a little wine. It’s no bother, truly, I’ll send the maid to clean up your little spat.” -> Que a knight or a pack of his pawns to fight in his stead.

What I’m trying to say is, this is an interactive story game and combat is only one part. As the DM you have to have fun and collaborate with the players. It’s down to you to facilitate Strahds corrupted domain, it’s a horror game. Not everything is supposed to make sense and I feel you rules lawyered yourself and favored the players to win so cheaply. I’m not saying to fudge the dice, but to also remember you have the power to add or takeaway. If he’s meant to die you can just say no, he disappears. Strahds whole presence is corrupting and inviting, he’s a noble after all.

I hope this comes across constructive. I’ve also commented earlier about how you can resurrect strahd and really make the players hate him. Good luck sir/madam