Unclear. It is larger than Mariupol, but it was not extensively prepared for siege like Mariupol was, and it does not have much of Ukrainian military forces, not anything like Azov's and the marine brigade (now fully eliminated).
I'd guess, however, that Nikolaev would come before Kharkov, and even that would wait for the endgame on Donbass.
That sounds about right. Seems like the Ukrainians put their biggest defensive effort into Mariupol, and rightly so given that the land bridge is the biggest objective of the Russians. Seems like the Donbass area is going Russia’s way so far, with that massive weapons cache they found just sitting there. Hoping this ends soon.