Invoker Avatar build - looking for general explanations about skill gems, Ascendancy nodes, damage scaling, sustain options and ideas about the build in general

Disclaimer: I don't need advice on what stats to look for in gear or how to farm or how to do either of my skill trees. What I want to do is explain what my idea for the build was and to hear what you think would help me make that build the best it can possibly be.

I made an Invoker and I wanted to make a build focusing around the Ascendancy skill Unbound Avatar. Logically, it made sense to me that the Ascendancy node that triggers Elemental Expression on melee crit would go well with that, but recently I've started doubting the usefulness of Elemental Expression. The spells were meant to be used only to apply elemental ailments, but since they are spells, none of the attack dmg increases do anything and I do not know how much increased chance to inflict elemental ailments would work when the spells themselves deal punitive damage.

I've been using Power Charges and Flicker Strike to do a large number of attacks using a Quarterstaff with as much elemental damage as I could make and am using Gathering Storm and Siphoning Strike to keep generating enough Power Charges.

I wanted to hear the ideas of more knowledgeable redditors here if there is maybe something I'm missing or something that could make this build more viable.