If Russia wants to fight for the Kurils, Russia has to be able to engage the Japanese at sea, not just curl up into a spiky ball like a hedgehog. That requires robust anti-submarine capabilities, the ability to put together at least a couple of taskforces of large surface combatants (at the same time). A large carrier with several fighter squadrons and fixed wing AEW would go a long way towards improving on that (a modern MPA would too). Alternatively Russia may be faced with the unfortunate situation of either having to resort to nuclear options or face a conventional defeat. On the flip side Japan does not currently seem aggressively inclined. This could always change, but it would be a quite different world, and a move like that by Japan would set all kinds of precedents and open all kinds of doors for other countries to act aggressively including China. And while Russia and China are certainly not formal allies, nor do I think China would go to war with Japan over the Kurils, China might see this as a good time to make their own moves.
Ukraine signed a whole bucket of naval contracts for small missile boats, patrol boats, and now corvettes. Is there money for all of this? Somehow I have my doubts. And while some countries may be generous with defense loans to Ukraine for political reasons, I'm not sold that the Turks will be. The Ukrainian navy today is a sad joke, hardly worthy of the name "navy". If they somehow fund this giant re-armament and field the ships, there's still the question of operating costs. We shall see what happens, but I suspect the Black Sea Fleet will gets its first 22350 before Ukraine gets its first Turkish corvette.
As for "countering" the Japanese naval threat, the Pacific Fleet isn't up to it, and won't be for a long time (possibly ever). Japan has one of the most capable navies on the planet if not in numbers, certainly technologically speaking. And their navy is mostly all in once place, not split between 5 theaters with wildly different requirements and threats. The Pacific Fleet definitely needs SSGNs and frigates, but current plans have them receiving 4 22350s. Even if they get all 4 by the end of the decade, as well as 10 total 20380s, and 3-4 Yasens, I still doubt they will be able to outfight the Japanese at sea. And again, those are optimistic plans.