I come at improvised music more from the blues and rock end of the world than the jazz end, though I am a casual fan of Charles Mingus.
Some of that can be a mess, for sure. If you gave me one word to describe Phish, it would be "amorphous", and I wouldn't mean it as a compliment.
On the other hand, The Allman Brothers Band made a lot of magic before Duane Allman and Berry Oakley died, much of it recorded officially or bootleg. Mountain Jam grew out of Donovan's First There is a Mountain. Which is worth listening to to understand where some of the phrases come from.
Mountain Jam (and their other longer songs) has distinct movements and part of the pleasure of listening to the various recordings is to hear the variations between them. The form remains pretty constant, but the details vary.
For my money, I think the July 5th recording at the Atlanta International Pop Festival surpasses the more definitive Eat a Peach version. The Ludlow Garage version runs to 44 minutes and isn't as focused. And the July 3rd recording at Atlanta is a treat because it was interrupted by a rainstorm. The second part picks up in the middle of the song after the rain. It feels a little more experimental. Those are all comparatively easily found on official albums.
Another treat is Medeski, Martin, and Wood. I'm not hugely familiar with their catalog, but Friday Afternoon in the Universe is an example of just how close the music can sit to the brink of disaster without going over. Like, if they weren't so damn good, it would end in a dumpster fire rhythmically, but they keep it together. It's a truly impressive listen.