Reddit mentions of Six Compositions (GTM) 2001

The Black Saint/Soul Note box is a very good suggestion, as it covers quite a range of Braxton's style(s): the second 'great quartet' (and variations) with Marliyn Crispell, the improv encounter (w Max Roach), the quirky approach to standards (in this case Monk), the larger ensemble and through-composed works (Eugene & No.173)...

For an in-depth look at his '70's work, The Complete Arista Recordings is great, if pricey. Dortmund covers a bit of that territory.

From the mid-'90's to mid-2000's his focus was on what he calls Ghost Trance Musics - maybe a tougher nut to crack, but extremely rewarding. Repetitive, meditative, hypnotic, addictive. It starts out pretty austere but ends up quite expansive, to over-simplify. The Firehouse 12 box, the one on Rastascan and many single and double sets on Braxton House are good examples.

The best place for more recent releases is Braxton House, Braxton's own label. Scroll through and sample these, a huge resource of great music. If you care to join the umbrella organization, the Tri-Centric Foundation, you get free access to a bunch of great bootlegs, as well as discounts on the official stuff.