Lenin was a Marxist when it was convenient, but not when it didn’t gain him power. Certainly after joining with Trotsky it gets very difficult to call him Marxist. Obviously the Mensheviks never considered him a Marxist, with mixed feelings from the SRs. I think following the machine gunners of kronstadt is the easiest way to tell when he’s being a Marxist vs something else. They were pretty reliablely Marxist the entire revolution.