The strategy is not even a secret. Russia sees itself as a major influence of the Europe.
You cannot control stable governments, so you destabilise them with various tools for prolonged periods of time and then you end up with a country which is much easier to influence.
But Russia is an aggressive authoritarian state that was already caught for (smaller) acts of sabotage in EU, some of them quite dangerous. Why they are doing this? Who knows, war in Ukraine was not rational too. Perhaps some people want to be evil just for the sake of being evil.
As a Russian emigrant, I long stopped trying to rationalize Kremlin decisions. Why authoritarians are authoritarians? Who knows. Mad with power or something.