There is no significant commercial port in Crimea (the main port is Odessa, which still belongs to Ukraine), and it does not need a military port: from the entire Ukrainian Navy in Sevastopol, by 2014, there were about a couple of ships left that could move independently. Crimea itself is also not particularly needed for Ukraine: during the years of its independence, it almost did not invest in this region, as a result of which it fell into decay. Ukraine does not need the peninsula even today: instead of investing money in it, it is much more promising to ask the West for money to “fight Russian aggression.”
Donbass is an economically more attractive region, but after the decline (in which it has come since independence from Kiev), it will also require significant costs from the meager state budget.
You underestimate the war in Chechnya. In terms of the number of victims on both sides, this was quite comparable to the war in the Donbass. At the same time, the financial costs of the Chechen war and the damage to infrastructure are probably much greater. But the main thing is that the loss of Chechnya would be the beginning of the destruction of Russia: at that time, separatism was gaining strength in different regions of the country.