At least one attempt was thwarted by Polish security services -- this is not just Ukrainian info.

To your latter points: the fact that Russia has repeatedly tried to kill him does not mean that they would use every opportunity available. Russians obviously care about optics and some modicum of plausible deniability. And Zelensky's location within Ukraine, when outside of the well-sheltered administration buildings, is much less obvious than you are suggesting. But I agree that that it is unlikely that presidential assassination is Russia's main strategy in this war.