I don’t see how this invasion is “rational” for Russia’s interests. Maybe I’ll be proven wrong, but right now it seems likely to strengthen NATO (and possibly expand it) and its resolve, unite Europe, increase the chances that most Ukrainians see Europe as a better partner and ally than Russia, and isolate Russia both diplomatically and economically. They might win in the short run, but what are the chances that this ends well for a not-terribly-rich country like Russa trying to invade and occupy (or otherwise massively change the internal workings of) a very large country?
Ukraine is bigger and stronger than Iraq was in 2003. Russia is weaker and poorer than the US was in 2003. How likely is it that this ends up better for Russia than the Iraq war of choice did for the US?