The war was never in Russia

s strategic interest and Putin basically took the worst option. The days prior to the invasion he recognized the Lugansk and Donbas regions as independent and sent in Russian "peacekeepers". Ukraine would have done nothing but complain about this-there would have been no war. Ukraine would have been humiliated and Russia would have "saved" the people from these regions. Sanctions would have been light or non-existent.

So that was Putin's first chance of victory.

The second is the fall of Mariupol, allowing the land bridge to Crimea. He could have claimed victory at this point.

The Third is taking all of Lugansk and Donbas.

All of these were potential off-ramps for the war, and if Russia ever called for talks at such time-the west would be pushing Ukraine hard to accept them. Western leaders know they can't fund the war forever, and soon won't even have the armaments for prolonged war-with Stinger missiles not produced in a decade and Javelins only produced at a rate of 1800 per year(and Ukraine using 60,000 of them). Plus the public, especially facing high inflation and gas prices- are losing patience and focus.

Every gain Ukraine has made, and territory it is now holding onto, is largely dependant on western munitions and aid. And last month the supply from Europe dropped markedly.

So Ukraine has to get some gains in the south quickly to keep the west enthused, and to get Russia to finally start to talk. Pressuring Crimea is the best way to do that. otherwise it will all be on Russia's terms, probably with Odesa taken too.

Putin is well known for claiming victory, he did it once before in 2014-2015 in Eastern Ukraine, then went onto Syria and declared victory there too- and neither were successes. So he doesn't actually have to win, he just needs to be able to sell things as a win to his people. The point is, he has had plenty of opportunities to do that already but seems to want more.