> Most NATO members in Europe vastly reduced their militaries and defense spendingMost western members did. Eastern ones, meanwhile, ramped it up very significantly. Which is the only reason Western countries could get away with it, really - NATO is supposed to feed the American defense industry.
> NATO only expanded east because of Russia's posturing
More like because of the bordering countries clamoured for protection, even when there was no threat whatsoever (since the Russian military was in utter disarray). In a way, they were right: the ones who didn't manage to escape Moscow's orbit back then (Ukraine, Belarus, the Caucasus stans...), are now enduring dictatorship or war. In another way, they fuelled the Russian paranoia that fed Putin's nationalism, eventually ensuring that the doomsday scenarios did, in fact, materialize.
> The most baffling part is literally no one wants to invade Russia.
It's not just about immediate military occupation. Gazprom is an important piece of Putin's kleptocratic powerbase, and Gazprom needs to be free of Ukrainian control of their pipelines. If NordStream2 can't happen, because the Germans have to obey their American lord-protectors, then Putin must establish an alternative to the South - which doesn't need to be entirely in Russian hands, but rather in a position where Russian troops can secure it in a matter of days if necessary. In my view this is the real endgame.
This, obviously, on top of putting the strategic Crimean ports in a safer position; because NATO might not want to invade Russia, but Erdogan's Turkish troops are not shy about "securing" stuff (any sanctions against them carving up Syria? Not that I'm aware of...).