It's important for Leftists to take an internationalist stance, so as to avoid perpetuating Imperialism like PatSocs seek to.

As for sources on numbers in different niches, I don't think there are hard numbers we can look at outside of viewing which tendencies have had the most traction and widest impact historically, which is currently Marxism-Leninism, especially if we include the CPC and assume a good chunk of its 96 million members are Marxist-Leninists.

I don't know what point you're trying to make about the Soviets with respect to "killing millions" or "banning opposition" outside of what I said, you aren't really pointing at specifics so there's nothing for me to respond to, other than to say the Black Book of Communism has long been debunked.

As for the Soviets, power was dramatically equalized, especially compared to Tsarist Russia and the Russian Federation. For Stalin, the CIA didn't think him a dictator. He certainly held a lot of power, but he wasn't unaccountable nor was he the one making all of the decisions. Same with Kruschev. That doesn't mean no Soviet leader has made mistakes, or had self-interested intentions, but at the same time you are taking an ahistorical, dogmatic view of the Soviet Union.

What you describe, with your heavy progressive tax rates, has only ever been in place in countries fearing a revolution while neighboring a Socialist power, historically the USSR. It's one thing to think a system would be nice, it's another thing entirely to shift towards it. Moreover, without addressing Capitalism, your "decentralization" is just an attempt to break up industry and keep Capitalism going a bit longer, like cutting your arm so it never fully heals.

Back to the Communists, I don't genuinely understand who you would support, it seems you let perfect be an enemy of good, which is just nihilism and passive support for the status quo.

Oh, you answered it in your next paragraph. It's no wonder you hold western-centric views, support for the Nordics makes that clear. The Nordics fund their safety nets through brutal loans and export of Capital, a process identified and tracked as Imperialism. They essentially function as landlords in country-form, expropriating far more value from sheer ownership of Capital than they actually produce, it's a form of usury. These Safety Nets are declining (as you yourself are noticing) because the Soviets are no longer right next door, pressuring the Capitalists in your country to offer concessions. That's why the Nordics are eroding.

I think a big part of your worldview is thinking the Nordics separate from US Imperialism, and not willing accomplices to the looting of the Global South. It might hurt, but you should look into the IMF and how Western Europe and the US work together to serve as global landlords, backed by the US's millitary and NATO membership as essentially a protection racket.