What is the quality of your Flourishing Strike in comparison to Lucky Gambit? I cannot see how LG would hit harder if the tier of the abilities is comparable. For reference, I took LG as my Celestial, and have Flourishing at Ancient and my top hit on Flourishing hits anywhere from 10-30T more (very inexact science due to constantly changing stats and fabled/mythical crits, but I cannot remember a named raid fight Flourishing was not my highest single hit)
You also are not casting 5 CAs in that 2.5 seconds. Not logged in to double check, but saying .5 second cast for most single target CAs, THEN add another .5 second recovery time before your next ability begins casting, so 1 second per ability(also the reason for asking for the increase in duration for DoM). Most of the rest of my single target CAs are current expert versions and no three added together come close to what Flourishing does.
For science, put it at the top of your rotation, giving it the benefit of all your temps, and cast it every time it refreshes and see what it parses.
I can dm you a Chromatic Chamber parse when I get home for comparison