I believe that it is somewhat useful to focus on the USSR, since they had a lot of valuable praxis and theory which a Marxist should learn, but I agree that one should expand their knowledge to other countries of actually existing socialism, and even other non-imperialist (yet not communist) countries

It is a simple one really: that imperialism brought civilization, technology, and relative prosperity to the natives (some youtube channel that "debunked" communism said this)

I read the ABC of communism by Bukharin and Preobrazhensky, which is a concise explanation of socialist theory, application, Russian socio-economic conditions at the time, etc.

Understandable, that is useful too

Of course! They have to create agents of capital from amongst the workers, and that means proclaiming the "benefits" of imperialism.

National Bolshevism makes no sense to me. What exactly is it?