I think its quite balanced. Its not immune to solution and has an upkeep.
A mechanical issue I have with it is the Overlap of the Shadowalks turning off Immaterial after 1 turn and permanent Immaterial from Quintessence. Actually, make the 2 issues as turnoffs are done when a creature has its action, so you will either not have Immaterial after the end of your turn or you need to add a new mechanic to allow abilities to roll over turns.