Since I started so late, I doubt I will be able to catch the top players but I figure if I play a lot maybe I will get closer.
Any thoughts on what builds help with T6 runs?
So far I don't believe I can solo T6 rifts but with a group I have been able to help and have been getting xp and items. I do notice that in certain situations my HP drops much more compared to others which still bothers me because it just seems like I spike more in HP compared to others.
I'd suggest you just play whatever difficulty you can, maybe a bit higher with some friends that carry you (or leech on T6 in a public game). Once you got your full set, check what weapon/shield you got, and check for a build for that in the build section, there are only so many viable setups.
On the "starting late": you got 3 pieces of your set in 114 paragon. The ring of royal grandeur is easy to get these days (t6 bounty bags drop 100% a legendary). You only need 2 more pieces to get going. In the next few hours you can have your entire build done and start competing with the "top players".