Regarding what imperialism even is, I think you should return to how I explained it as a system earlier:

Imperialism as a stage in capitalism is best characterized by consolidation of monopoly bank and industrial capital into finance capital, and the shift from an economy based on export of commodities to export of capital. Just because the US does this does not mean this is how imperialism was first analyzed, nor does trying to vulgarize the definition of imperialism to be any aggressive action by one country to another erase the fact that this system does materially exist. It exists even if you shut your eyes to it.

In essence, imperialism as Marxists understand it is an actual system that materially exists, no matter what we call it. We can call it "Marxist imperialism" if you like, and we can use "imperialism" to refer to the vibes-based understanding that exists predominantly. No matter what we call it, Marxists specifically oppose "Marxist imperialism" while we don't actually believe everything that falls under colloquial understandings of imperialism is inherently bad, such as when the Statesian north invaded the Statesian south.

As for the Warsaw pact vs. NATO, this is an excellent example of why understanding "Marxist imperialism" as a system is so critical. Not all military alliances are tools of imperialism. The key difference between the Warsaw Pact countries and NATO is that the Warsaw Pact countries were all socialist, and not "Marxist imperialist," while the NATO countries all are "Marxist imperialist" and use NATO to further this system as described earlier.

Regarding Georgia and Ukraine, neither represents an expansion of Russian finance capital. Most of Russia's military action is a response to encirclement by imperialist countries and militaries. The west throws allegations of colonial intent despite provoking the invasions in the first place. The goal of the west is to isolate and Balkanize Russia so as to make it easier for western financial capital to proliferate, while Russia is primarily self-interested in maintaining its own sovereignty.

The major difference between Russia and the US is that Russia is a nationalist dominated capitalist economy based on commodity production, while the US is an imperialist dominated capitalist economy based on export of capital and unequal exchange. Both are dominated by capitalists, but the actual material functioning of these economies is very different precisely because of the difference in stages in development. This puts the two at odds.

As for the dissolution of the USSR, it was dissolved via counterrevolution, not because socialism is less effective. There are complexities there, but these go beyond the extent of this conversation. What's important is that socialism dissolved, but this did not bring a resurgance of "Marxist imperialism," because the world was already divided up amongst the existing "Marxist imperialist" powers.

Again, when you equate Russia and the US, and think Russia could ever replace the US as imperial hegemon, you make it clear that you do not understand "Marxist imperialism" as a system. I will restate this again: Russia has an absolutely paltry sum of finance capital. Without developed finance capital, Russia cannot replace the US Empire as imperial hegemon. "Marxist imperialism" requires the very tools and functions of "Marxist imperialism" to exist in the first place, and these do not show up instantly in other countries. Russia has less than a dozen of the top 500 countries in the world. It has no ability to replace the US Empire.

As for Russia not having the "right" to attack Ukraine, this is a moralist argument, not a materialist one. Russia is self-interested in its own defense, and the west knowingly poked the bear. The largest victims are those in the Donbass region that were ethnically cleansed by Ukraine, and the second largest are the members of the Ukrainian working classes that are being sent to their deaths so that the west can harvest Ukraine for minerals while the comprador regime in Kiev profits on the side.

So you’re really going to pretend that the Warsaw Pact wasn’t imperialism? Even though you claim NATO is?

NATO is carrying out a genocide and enforces relations of severe unequal exchange, unlike Warsaw Pact which literally acted against all that.

What is your issue with the Warsaw Pact?

Not for lack of trying. If you don’t think invading Georgia and Ukraine count as attempts at colonization

Apart from you not understanding what colonisation is, there are also the following issues:

1. Georgia was the one to attack the peacekeepers in South Ossetia, as well as South Ossetians. Not sure what you think Russia should have done differently.
2. Ukraine had a NATO coup d'etat, and immediately attempted to become a glorified NATO military base (while also selling off its wealth to NATO capitalists in unpopular moves). That is an unambiguous act of aggression, and I am again not sure what you think Russia should have done differently.

There is essentially fuck-all difference between Russia under Putin and the US under Trump

Apart from the fact that, for all its massive faults, contemporary Russia hasn't been the aggressor in military conflicts (no, reaction to NATO's aggression via Ukraine is not itself aggression), and has militarily helped various countries fighting against your colonial subjugation of them.

Also, why are you singling out the US and Trump, and not all of the other invaders of Iraq and Afghanistan?

I understand what you think you’re going to accomplish by supporting Russia, but all you would actually accomplish is trading out one imperialist capitalist oppressor for another.

Let's say you were correct. Why wouldn't one trade NATO for a much more manageable foe?