Correct, but none with a Slavic majority. Similar with China. Lots of democracies. None with mostly Han Chinese.
The threat to both comes from the disabling of their argument that democracy doesn’t work for “their” people. It’s difficult for Russia to define a set of people that encompasses its population but excludes Ukrainians. Same for Beijing with respect to Hong Kong and Taiwan.
China cared about Taiwan when it was a authoritarian dictatorship for most of its history. I don't think it's a matter of government but a matter of lost territory. Using your logic, the north was worried about the functional democracy of the confederacy.
Even if taiwan adopted communism, it wouldn't matter because it's a matter of territorial integrity. Meaning, it's just as lightly that we'd let south carolina secede than china would allow taiwan to leave.
As for russia, considering that it's a democratic republic, I don't think they are too worried about that. What they are worried about is the geopolitical issues with ukraine - especially foreign interference in ukraine.
This is actually false. The reason Russia and China care so much about Ukraine and Taiwan, is because they worry having a functional democracy next door may destabilize their own regimes.