Putin did not expect it to be a fight at all. The most damning evidence of this is that those columns that were blitzing towards Kyiv early in the war had Rosgvardia units in them, the whole purpose of which is to suppress street protests etc - and they even packed their riot gear! This is the kind of thing you do if you expect the city to fall right away, and your sole problem then is simmering dissent.
Comparison to Germany in 1930s is fairly straightforward. Both Germany and Russia believed that they have suffered considerable humiliation in the last major conflict (cold war for Russia), lost "their" lands in the process, and were plunged into an economic black hole. In both, this triggered an embrace of irredentism and the overall ideology that can be summed up as "we'll be back, and then you'll be sorry". Both produced increasingly authoritarian regimes that, as they solidified their rule, started to look more and more at "their" lands among their neighbors. Both accused one of their neighbors of persecution of "their" ethnic minority, and justified the invasion and occupation of said neighbor on those grounds.
As for WW3 - no, I don't think it'll start over Kazakhstan. But if you wait long enough, it might start over, say, Latvia. The problem is that, with every country that the collective West allows Russia to overrun, NATO Article 5 is perceived as less of a real deal and more of a bluff. Basically, if US is unwilling to fight for Ukraine for the fear of escalating into nuclear, why would it fight for e.g. Latvia when the likelihood of that escalation would be even higher? Because Latvia is a NATO member? American isolationists will say that NATO isn't worth getting nuked over anymore so than Ukraine. So if you "draw the red line" there, you might find that line rather faded by the time Russia comes close to crossing it. They might cross it simply because they don't believe you have the guts to do anything about it by then.
Adding to the problem is the overly rosy picture of Ukraine successes in this war so far. Don't get me wrong - they are doing very well, exceeding all expectations by a large margin, and have plenty to show for it. But, at the end of the day, this is still a country of 50 million people with a $6 billion defense budget (2021) fighting against a country of 150 million with a $66 billion defense budget. Ukraine has had several mobilization waves since the beginning of the war, while Russia is still relying mostly on contractors and PMC. Ukrainian industry - which is largely in the east - is already devastated, while Russian manufacturing capacity is still fully intact, limited only by sanctions. If Russia were to truly switch to war footing to the same degree that Ukraine already did, Ukrainians won't be able to hold the line - not because they're bad at it, but because Russia can afford to overwhelm them with bodies and munitions. So you can't just assume that if we sit and wait long enough, the Russian advance will stall indefinitely, and that'll solve the problem long-term because all forces will remain tied in Ukraine.