If Ukraine really wants Crimea (and from all reports they do) - then I don't see how pushing all the way from the North/East through the Donbas and leaving Crimea for last makes that easy. They would not only have to leave behind forces to prevent a re-incursion from the Russian border, but they also would effectively push forces into Crimea which would allow Russia to defend it better.
Maybe a better route would be to take Kherson and Melitopol, then Crimea. After they take Crimea all they'll need to leave behind is a token force to hold it. They could then squeeze the Russians from the North, West, and South with everything they have then hold the border.
Then again, maybe they just want to soften up Crimea for a long time with arty and only need minimal forces to actually take it (perhaps lots of special forces type actions behind the lines).
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." - Ben Franklin