Eh, objects and constructs are immune to psychic damage, it is not that great. Plus the spell you are basing it on, primal savagery, isn't great to start with. I think that as it is, not many players would choose soul knife over mage hand or minor image.
More appealing to me would be 1d6 or 1d8 damage plus a thematic effect. Not sure what though.
Or perhaps, 1d4 psychic autodamage with no attack or save?
Fair point. If looking at Shocking Grasp as a point of comparison, you get the rider of taking away reactions and advantage if the creature is made of metal/wearing metal armor. So then the question would be, what would be an interesting rider for Psychic Blade? Ideally it would be thematic without doing something that has already been done.
One thing that came to mind is disrupting concentration. It is completely within the realm of such an ability. So perhaps the cantrip can be changed to this:
Psychic Blade
Evocation cantrip
Casting Time: 1 action
Range: Self
Components: S
Duration: Instantaneous
You channel your will into a weapon that can attack a person’s mind. Make a melee spell attack against one creature within 5 feet of you. On a hit, the target takes 1d8 psychic damage, and the target has disadvantage on any concentration checks to maintain an ongoing spell until the start of your next turn.
The spell’s damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8).
My only concern with this rider is that it seems so limited. It seems like a good rider to equate to the advantage shocking grasp grants against armored opponents. Perhaps also preventing reactions also makes sense?
So maybe a better option would be this write-up?
Psychic Blade
Evocation cantrip
Casting Time: 1 action
Range: Self
Components: S
Duration: Instantaneous
You channel your will into a weapon that can attack a person’s mind. Make a melee spell attack against one creature within 5 feet of you. On a hit, the target takes 1d8 psychic damage, and it can’t take reactions until the start of its next turn. If the target is hit and maintaining concentration on a spell, then target has disadvantage on its concentration check.
The spell’s damage increases by 1d8 when you reach 5th level (2d8), 11th level (3d8), and 17th level (4d8).
For an extra ribbon, what about not dreaming? Make them immune to effects involving dreams, like the dream spell or a night hag's haunting.
I had considered this direction as well. It certainly fits. Probably better than what I suggested with Inertial Armor. I think I'm going a bit too psionic warrior in my considerations of them, when that is not what they are. Just how I'd like to use them, which doesn't help in designing them based on lore.