well, crimea was Russia for many years, they can use that excuse... and it have 2 critical key values for Russia, that can be enough to justify a nuke for some people:
1- the Sevastopol port
Russia have 4 main ports: Sevastopol, Murmansk, Saint Petersburg and Vladivostok. Murmansk is too in the norh, Saint Petersburg is very important, while a little locked in the Baltic sea, Vladivostok is in the other side of the world. Sevastopol is also a little locked in the black sea, but is in the south, so serving a totally different region and can work all year. Military, as this war show, may have been downgraded by missile range and drone range, but it is still important as all ports do suffer from that problem.
2- the ketch strait
The strait that exist between Crimea and Russia control the access to the Azov sea. That in turn control the access to the Don river, Rostov-on-Don and all the rivers and channels that fuel the inner Russia trade. Ukraine being friendly, it would not be a problem, but with the Ukraine target of join NATO, the control over those vital russia blood line would be in danger. Block those and the alternative is Saint Petersburg... that also have NATO countries controlling that access... Vladivostok is on the other side of the world and Murmansk too north and far away to serve as alternative. So even if not in military danger, just the risk of mostly landlock Russia is a concern. That is why Peter the Great wanted the Saint Petersburg port in the north and Crimea and control of the Azov sea for the south port, he could see that only with those sea access Russia could grow, have good ports and good internal transport, most by boat, to the inland cities. Stalingrad, now Volgograd controls the Volga river that feeds a good part of western Russia... that was why the Soviets tried so hard to keep control of it. Don river connects to the Volga by channels. AFAIK, you can reach Saint Petersburg by boat sailing from the Azov sea via all the rivers and channels
3- as a bonus, the all Donbass give land access to Crimea and better protect Azov sea, so while the Bridge that connects Russia to Crimea helped a lot supply Crimea, the land connection via Donbass is still much better (and also supply water to Crimea)
So Crimea is important to Russia... how important is up to the leaders at that time... I would give away Crimea for peace and would try to create a economic USSR (without the communism) like EU... A stable free commerce and border between all those lands, to decreasing border problems and grow... and in a few decades, merge all that or one by one with the EU and create the wide European Asia Union. NATO expansion IS a problem... create a "NATO/Russia protected" treaty, where both agree that certain countries are neutral (Ukraine, Belarus and probably the Baltic states and Finland) and defended by the other side if ever attacked (ie: NATO, but without having either side solders or equipment in those countries... or joint military bases.. whatever is needed so each side check and trust the other)... or also join Russia to NATO, problem solved! :)