Trotsky was extremely irresponsible for talking about overthrowing the Soviet state the way he was. He was an extremely well respected original Bolshevik with lots of inroads into the Red Army, he had no business talking about overthrowing the state while it was besieged on all sides by forces of reaction and imperialism. I like Trotsky but for this alone I think Stalin was justified in running him out of the country and being suspicious. That said, the "Trotskyist plot" and the various purges and show trials of old Bolsheviks like Bukharin were completely unjustified and absurd and as far as I'm concerned defending that shit is where you cross the line from pragmatic Marxist-Leninist to Stalinaboo. His indiscriminate destruction of the Bolsheviks coupled with Lenin's thoughts about not trusting Stalin to be in too high a position of authority in the Party is quite an indictment imo, and too often dismissed as just another obnoxious Trot claim (of which there are plenty, this is not one). I have a lot of admiration for Stalin in his defense against fascism and general capability as an administrator, and especially for his life before being General Secretary, but not believing bourgeois propaganda about him doesn't mean we should unconditionally defend him.
As for "socialism in one country" I dont think there was realistically another way to go about it. And I don't think this constitutes any kind of reactionary nationalism – the USSR was made up of dozens of nations that to various degrees enjoyed autonomy and preservation. There are certain circumstances like the landing of nomads and the forced resettling of Siberian tribes that are objectionable, but basically I dont think there is a contradiction between internationalism and pragmatically building a socialist society within a set of borders de facto maintained by imperialism whether you like it or not. Cuba has done incredibly well for itself under those conditions, all things considered. Some say the DPRK has also done well in that sense but I don't know enough to comment. Tho I think socialism in one country was a good policy I do also think the USSR fucked up a lot of chances at internationalism. There is absolutely no reason that Yugoslavia and China should not have been supported as full allies. Stalin's realpolitik extended to reactionary states besieged by imperialism, but not to socialist ones? I don't understand it. A closely allied USSR and China could have been what ushered in a new century of socialist revolutions.
It's strange to me that people consider Trotsky less of a "tankie", he was a Red Army commander that oversaw the destruction of anarchists in Krondstat and Ukraine and breaking up of strikes, and advocated for the immediate collectivization of kulak land way before Stalin warmed to the idea, which he initially considered too extreme. I know the incident with Hungary is what led to the ☭TANKIE☭ label but afaik now it just refers to these sort of strong-armed "state-socialist" tactics of which Trotsky was absolutely a part.