I would prefer being able to click a treasure directly after playing Plaza to discard it. So I would make both Action cards and Treasures being clickable. Treasures could have a different border color to not confuse discarding with playing in the buy phase. (If there was no action card left in hand, one would have to click "end action" though, if one doesn't want to discard to Plaza.)
One problem with that: Crown.
Crown is already really tricky. Half of my "undo's" have been because I played crown as my last action rather than my first treasure.
Right, with Crown you have 3 options, so you would need a "don't discard" button.
I haven't played with Crown yet, but if playable cards had a differently colored border (blue?) in the buy phase, it might be less confusing in general.
One could log now, in how many cases a Treasure is discarded to Plaza. My hunch is, that it's still the majority.