Desalination is incredibly inefficient and not verysustainable. Creates extremely toxic and sometimes highly radioactive waste.

True, though from the Russia perspective that could've been seen as a potentially huge waste of money/resources.

Russia's difficulties resupplying Crimea would've gone beyond just water and effected them militarily, so if Ukraine decided to invade and retake that territory, then Russia could've been in a pretty difficult situation trying to hold it. At this point, there's only a bit of evidence to suggest that Ukraine was going to imminently invade Crimea, which is that Russian/DPR forces claim to have found premade victory medals for an upcoming Crimean campaign in Mariupol (having victory medals made before actually winning isn't unheard of btw).

Now idk if the claims about victory medals are actually true at this point, but it's very likely that Russia has considered a Ukrainian offensive into Crimea to be a very real possibility at some point. So with that in mind, putting in all of the effort and resources necessary to make a desalination plant in Crimea would look rather pointless if you think that there's a good chance the Ukrainians are going to invade and possibly retake the peninsula. So from that perspective, invading Ukraine first, taking the canals and establishing a land bridge to Crimea starts to look like a much safer and more efficient option compared to building a desalination plant and hoping that Ukraine either doesn't invade or that you can hold them off if they do invade.