MAD is still in place. Both the colonies and Russia have each on their own enough nuclear weapons to sterilise the planet several times over.
The colonies are no threat to Russia’s sovereignty.
As a defensive pact, NATO is not a threat to Russia’s sovereignty.
None of its direct non-NATO neighbours are a threat either.
He knows that.
Except maybe China.
If you think about it, that is likely why he plays nice with China (because they actually might be mad enough to pull the trigger), and antagonises the west, as he thinks he can push them around with little consequence.
If it is Ukraine’s resources he wants, for the “prosperity of Russia”, he could get them through peaceful relations and trade agreements. But of course Russia’s prosperity is not something he actually cares about. The rampant corruption and poverty in his country, and the concentration of wealth among a small group of oligarchs attests to that.
So really, if he had rational goals, the invasion of Ukraine is not rational.
No, what his goal is, is the reestablishment of the Soviet Union, probably under a different name, for his own personal glory, and so that he can leave his mark on history.
Hence his ludicrous demands of “security guarantees”, which prominently featured the demand that NATO remove all troops from former soviet states, current NATO members included, so that he would have an easier time conquering them later. (Of course, after fomenting rebellion by seeding and funding insurrectionists like in Ukraine’s separatist regions, which, lacking NATO forces, would have an easier time toppling extant governments and installing pro-russian puppet dictators like Belarus’ Lukashenko, who would then initiate the countries’ withdrawal from NATO, opening the countries up for either vassalisation, or outright annexation)
And the real answer to the question, which course of action would have the least chance of ending in global nuclear fire, is neither invading Ukraine before NATO admission, nor invading Ukraine after, but to not invade at all, and instead establishing peaceful relations and mutually beneficial trade agreements. That would have been the “rational and thoughtful” course of action.
Which he did not take, because in the end, he is not.