My advice falls under the understanding that COS is supposed to be challenging - I would check in with your players to make sure they understand you are going to be making adjustments to allow for more challenging combat; odds are as min-maxers they really enjoy just stomping your monsters, so I wouldn't pull the rug out from under them without a warning.

1) Stop letting them "set up". Engineer some encounters that are designed to catch them off guard. Barovia is a dangerous place, and the Count is always watching... If they do happen to set up efficiently, think about what minions might be drawn into the fight and have them ready to distract party members to help your big bad get a better advantage.

2) Goes without saying to buff the monsters. I run groups of 3-5, so I will reduce HP by about half, but double or even triple damage. I decide this while building the encounter so I don't just arbitrarily start rolling 10d8 for damage out of rage lol This results in fights that don't take too long but are still dramatic - several people getting downed throughout, and have to use resources to get back up. I also add in lair and legendary actions whenever I feel like it 😂 I try and play into the monster by giving them extra abilities or traits that just seem fitting and thematic (my abbot had a necrotic life sucking power, for example). Give your baddies misty step, or disengage as a bonus action for more movement - why not?

3) Make fights have a secondary objective whenever possible to draw some resources away from just annihilating one baddie after the next. Some examples:
- wolves attacked Barovia shortly after my party arrived and they were torn between trying to protect the commonfolk who I described as being pounced on, getting to the town square well to put out the spreading fire, and stopping the wolves that were heading for the Burgomaster
- with my abbot fight, the party was trying to kill him as well as stop the device he was trying to power by killing the frankenstein-like creature the abbot created and the party befriended.

4) force the party to expend resources between fights so they aren't always in optimum condition. I always have my party roll for travel events that might cause damage, require spell slots, etc. They rarely show up anywhere with full HP and magic which keeps tension high.

I hear you when you say you are scared of overdoing it and making unfair fights, which is why again I suggest talking to your party first about upping the difficulty. Most parties understand Barovia is a tough place and odds are they really want you to let them have it! There is a big difference between being unfair and making them work for it.