I like that suggestion of being able to choose which Part of C’Thun to use, but can’t use a Part you’ve already used.
1 suggestion I had is that C’Thun starts in your deck from the beginning with the same effect as the regular (6/6 spreads 6 damage) so you can draw and play it without needing to draw all Parts though for a weaker effect. In return though, I also proposed it doesn’t get 6 damage per Part, but only 3 and the other 3 is put in its stats (so, it goes from 6/6 spreading 6, to 9/9 spreading 9, all the way to 18/18 spreading 18). Because in this state getting 6 damage per Part would probably be too much.