The Russians could have (and indeed should have) just taken Donetsk and Luhansk and protected the waterway if that was their goal. Their goal was (and is) not

The Russians could have (and indeed should have) just taken Donetsk and Luhansk and protected the waterway if that was their goal. Their goal was (and is) not that. Their goal was the destruction of Ukraine as a country, and it's forced reunification with Russia as a foundational step in the re-emergence of a Russian empire. If you read Chinese defense arguments about Taiwan, they basically make the same hypothesis. Control of Taiwan forces the strategic sublimation of Japan and Korea, and re-establishes a Chinese empire. Dictators don't come up with new ideas.