I'm not so sure about the "mud season" hypothesis. Classic tank warfare of WWII, with a wave of tanks rolling abreast across farm and field may longer be effective or needed. Or desired by the aggressor. Modern highways, roads, and railways make it possible to bisect a country fairly quickly. The Donbass Region, which has been the area of continued small scale war since 2014, could be severed off by controlling only 140 miles of highway and several bridges over the Dnieper River.

In my observation, Putin likes to take chunks. He took Georgia, there were sanctions and out-cry but he still controls it. Same with Crimea. He might take the Donbass then pull back, regroup, start again in five years. But controlling Donbass gives him a lot more control of the northern coast of the Black Sea where there have been minor naval clashes in recent years.

The Donbass also has extensive coal reserves and industrial centers.

(edited)