If by "nothing required Lenin to purge other fellow socialists" you mean there weren't counter-revolutionary Mensheviks and other such assets in positions of power during a literal civil war,

Oh the good old, "Call them counter revolutionary and now it's okay to shoot them". The Mensheviks and Socialist Revolutionaries had a far bigger hand in the Febuary Revolution than the Bolsheviks, and later had their legitimacy confirmed with a free election. If anyone was counter-revolutionary it'd be the betrayers of the majority elected socialist government, the Bolsheviks under Lenin.

If by "crushing syndicalism" in Russia you mean not immediately giving the means of production to uneducated workers, but instead slowly growing unions to unforeseen levels of participation,

Nice try, but the Union's under Marxist Leninist regimes are always heavily policed and controlled by their respective governments. "In this respect, through the Western lens of a dichotomy of independent unions versus company unions, they were more accurately comparable to company unions, as "unlike unions in the West, the Soviet variety do not fight for the economic interests of the workers. They are conveyor belts for Party instructions, carrying punishments and rewards to industrial and collective farm employees. Soviet trade unions work with their employer, the government, and not against it." I'm not completely against planning, but I'm against banning any form of independent organization/representation for workers, and then striking them down whenever they complain.

If by "crushing the working class" you mean creating unforeseen levels of access to healthcare, education, eliminating unemployment and homelessness.

While the soviets did eventually institute some rather progressive welfare reforms for their time, these reforms had no popular demand from the working class that grain requisition did originally, and were put in place from above, not gained through demands of a working class movement. I believe most Leninists would call this a Bourgeoisie Concession had it happened in a capitalist country.

The Bolshevik revolution faced a coalition of the Tsarist loyalists in the civil war, which was militarily and economically supported by a total of 14 other countries, including Britain and its colonies, France, and many other European powers, in the direct aftermath of WW1

The Bolsheviks only faced an allied invasion after having pulled out of the war and signed a separate peace with the Germans through Brest-Litovsk, which happened after the bolsheviks had overthrown the mensheviks. More importantly though, the civil war didn't even start until the Bolsheviks had crushed their socialist opposition. The Tsarists had almost zero power and were thoroughly crushed after events such as the Kornilov affair, and it would've stayed that way had the Bolsheviks not fractured the Russian socialist movement.

Of course there were. Stalinism was extremely excessive and brutal during WW2, and the oppression went way overboard.

I actually have far more of a problem with Stalins actions in the 1920s before the Great Purge and World War 2. Between Trotsky, Bukharin, Zinoviev, Ryhzkov, and their supporters, Stalin had effectively crushed any opposition even within the party. Not though discrediting them intellectually or by testing their ideas and showing the failures, but instead by killing or imprisoning them.

I thoroughly believe these conflicts during the 1920s were what doomed the USSR, Stalin had killed almost all competing ideas for a potentially better Socialism, even among fellow Leninists. And through his purges Stalin became the gold standard of Leninism for the USSR, with the effective intellectual ban on the ideas of Bukharin, Zinoviev, and Trotsky lasting until the end of the USSR.

Let's learn from the mistakes of the past and build more fair and resilient systems that won't commit those excesses, or will minimize them.

That is what I tried to do here, and is why I'm majoring in Soviet history, but I don't think just quoting the old propaganda justifications or Parenti counts as sober analysis. As socialists I believe we should focus historically on the possible off-ramps from authoritarianism that the USSR and all "Actually Existing Socialist" states had. Which is why I focus on Stalin and how the collection of power into Lenin's position lead to the possibility of much larger later abuses under Stalin, imagine how much could've changed had after the civil war the Factions ban been lifted and the working class was allowed to choose between the Workers Opposition, Left Opposition or Right Opposition in free and fair internal Party elections like takes place now in most modern Socialist parties. I'm sure in this scenario Stalin wouldn't of been able to commit the atrocities he later did, and that Soviet politics would rely much more on the will of the working class than behind the curtain political maneuvering.