Isaac Deutscher’s first book in his a trilogy on Trotsky is good. There might be parts of the narrative framing to disagree with. But he brings the factions to life, IIRC.

(It was one of the first book that helped me take Marxism seriously, because it shows that Marxists are not the hive-mind totalitarians they’re portrayed as. Whether you’ll get the same if you’re already an ML… I’m unsure. It’s about a later period but at a similar time I also read Tariq Ali’s Fear of Mirrors while in Germany. That was… eye-opening and made me rethink European and Soviet history.)

Edit: you might also question the framing, and I’ve yet to read it, but China Miéville’s October might be useful. I’ve heard it criticised for being too much of a recounting of internal Soviet politics (rather than the quasi-novel it’s supposed to be), but that might be what you’re after.