Your first paragraph is exactly why the Czechoslovakia example is a propos. Putin has been viewing Ukraine as his implicit property, just like the USSR thought it had legitimate claim to Czechoslovakia's internal affairs. When the smaller countries "fell out of line," the USSR/Russia invaded.

As to why there's no guerilla warfare, I highly recommend "20 days in Mariupol". Russia is a brutal occupier (not to mention that a huge share of residents fled, either in 2014 or in 2022).