Regarding the Astral Construct, I think both its HP and its damage output (Specially at the higher levels) is too high, even if it only lasts for a Minute and uses up concentration. I would Start at 4d6+4 HP and go up from there.
Quick note: Astral Construct should be up to 1 hour like the other conjure spells. I had it there before, but it somehow got reverted.
The HP and damage values line up with the expected values of monsters based on Challenge rating. If you compare the 5th level astral construct to the
Conjure Elemental spell you'll see they are quite similar:
Earth Elemental: Challenge 5, 126 HP, 17 AC, damage resistance to b/p/s from nonmagical. Does about 28 damage per turn if both hit.
Astral Construct (5th level): Challenge 5, 112 HP, 17 AC, condition immunities. Does about 28 damage per turn if both hit. Enhancements for either more damage or utility.
Beyond that I compared the astral construct at every possible power level. See
Spell Balance. So by those numbers the 5th level Astral Construct is nearly identical to
Conjure Elemental.
Is there a specific level that you think is too strong?
Also, I don't think the psycristal HP should scale with manifester level, as the wizard's familiar HP doesn't. It would the psion's "familiar" simply better than the wizard's, I wold give it 12 CON and have its HP be just 1d4+1.
The reason a psicrystal has more hp is
fusion. Fusing with your crystal is a decent HP buff. Each familiar has unique and interesting features.
Most can make a small attack (probably for utility). A bat has echolocation and keen hearing, a cat has keen smell, a frog is amphibious and can leap far, an owl has flyby to deliver spells.
I'm not saying a psicrystal should 100% have the hp, but if I were to remove HP that would reduce the usage for
fusion and I would need to add a different feature.