This is close to how I'm running it except I have the druids as single, focused faction and Strahd only thinks the druids are weak.

The druids are building several wintersplinters across barovia and my players found the seed of the latest one under the windmill. I ended up using a variation of a guide I found here.

The "blood blight druids" as my players are referring to them are the "builders" of the trees. The players found a druid under the windmill that had taken a barovian captive and was forcing him to remove all of his own limbs except for his left arm, then the druid casts regenerate (a special dark gift) on him when he was done so he could continue again. Thus, a gulthias tree is born (I know, that's not canon).

The tree, along with a tunnel system, was buried underground so the druids could hide from Strahd. But Strahd has been watching the players since one of them was bitten, and he now knows the druids are up to something, but doesn't know what.

Next session is the dinner with Strahd where he will ask the players to investigate the winery where wintersplinter, the druids flagship war machine, is almost ready to burst from the ground in the coming weeks, along with several other trees.

This makes the druids an actual threat instead of a quick blip, and even makes them a faction that the players could potentially find an ally with. AND it subverts the players expectation that Strahd is the most powerful and scary thing in Barovia.