If you check the airports around Odessa and Crimea they was reufurbished recently to be cappable to accept heavy military planes.Crimea was a totally different scenario.
The Ukrainian government was in shambles due to the Maidan coup which represented a sudden (and constitutionally illegal) change of government from a pro-Russian to one controlled by Anti-Russian parties. Furthermore, since the Russian navy was already leasing Sevastopol, so their personnel were already there.
In the current time, the best case scenario in the Russian calculation would be collapse, but to me, I don't think they were counting on it. The signs of a willingness to wage a long resistance were pretty clear.
In the big picture, I think Russia/Putin felt he was running out of time. US was continuing to pour in weapons and training. Zelensky was ready to sign on the dotted line for a number of Chinese infrastructure projects (including projects in Mariupol and Berdyansk) after pushing them away early on to try to engage with the West. If he waited any longer he risked the Ukrainian military being strong enough to sweep away the DPR and LPR, or worse, threatening Chinese economic interests which risks his only friendly relationship.
I presume the Russians really run out of time , most likelly by the end of this year Ukraine would be the home of several USA bomber and airforce squadron , if there is no "special operation " .