Double-tap should be a last resort for your least-used skills. I have Swiftcast, Limit Break and Repose down there, and a few things like target markers and waymarks.

The L2>R2 and R2>L2 hotbars are a much more natural choice for your second hotbar.

AST cards are frequently used and I put them right on top, on the left side of the L2 hotbar, where I can easily shift across to them between other skills.