So I've been spending way too much time on Reddit, reading everything I can find about M6 and Sentries.
Based on
this link, it seems you get the most bang for your buck by either having CA+MS+EA or CA+MS+Imp as your spenders. (I say "based" because after comparing seemingly all the possible combinations, that's the two you can pick when doing the simulation)
And also, on
this spreadsheet, you can see what would be considered BiS stats per item. Annoyingly, I have gone to great lengths to get +CA damage on all my items, when I should have gone for MS. Damn!
On a side note, I've found a fun build and gear combo for just farming bounties and normal rifts on T6. It obviously won't work on greater rifts, but for everything else it's a lot quicker. I remove Entangling Shot for Chakram - Twin Chakrams. I swap out my Bombadier for a Spines of Seething Hatred (Chakrams now generate Hatred). On T6 I find I rarely have time to get out more than 2 sentries before everything in an area is dead, and between myself and my max 3 sentries, the entire area is covered in chakrams in a matter of seconds. It's fun, and at least a bit more interactive than the normal "drop sentries and run like a little girl" approach.