What happened in the 90s and 2000s then? The west had been propagandized for half a century that Russia was the enemy, so had Russians, so neither side was going to hold out there hand and try for cooperation.

Both sides did, actually. The refrain in the US was always that Russians weren't the enemy, that Communism was - we learned our lesson with our propaganda campaigns against Germany in WW1. Even Reagan, anti-Soviet agitator extraordinaire, stuck to that line. Yeltsin expressed strong interest in joining the international order - even Putin claims that he raised the issue of NATO, though he wanted special fast-track status for Russia, which was a non-starter. God, I don't know how to express the sheer triumphalism of 90s academia on the subject. Communism was defeated. Russia was free. That was what the West told itself.

If the west was serious about incorporating Russia they would’ve done some sort of marshall plan to modernize them before bringing them into the fold like Poland.

There was no Marshall Plan for Poland. Or any of the ex-Sovs or Warsaw Pact states. The Soviet Union refused all aid when the Marshall Plan was implemented in the 40s, and there was neither the political will nor interest in the 90s in extending generous aid terms. Poland, Czechia, Slovakia, Hungary, Romania, Bulgaria, Ukraine, the Baltic States - they all managed to escape not only the horrendous effects of 'Shock Therapy', but also half a century of Soviet exploitation, if not necessarily evenly. Yet Russia, and Belarus, did not. One might then begin looking for reasons why Russia and Belarus failed to integrate, rather than reasons why the West failed to welcome, with that in mind.

They didn’t, they did let a bunch of their business men buy former public property for pennies on the dollar, which I guess is investment but only really for the corrupt officials who got rich off it.

This I agree with. 'Shock therapy' was nothing but a plundering of the old Soviet states by a plutocratic elite who could generously be said to have gotten high on their own supply of free market fetishism, or more cynically to simply have done what the faithless dogs would do with any weakened state - strip it bare for short term gain.

There was no path to EU membership, especially after Poland and the baltics joined because they, justifiably, hate Russia for all the imperial oppression they’ve done over the centuries.

Man, you put too much stock in old grudges. As much as I enjoy ragging on Old Worlders for their ancient blood feuds, it's not actually that prominent in terms of diplomatic behavior. International relations are predicated almost exclusively on "What can you offer me NOW?" If they weren't, Germany would never have snuggled up to Russia, France would currently be blockading Britain, and Spain would have cut off the New World colonies which kicked them out. In the 30s and 40s, Ukrainians and Poles were genociding each other, independent of Nazi-led initiatives - now the relation between the two countries is very warm.

History matters for context - but speaking as a History Major, it rarely matters more than present circumstances.