Is sorcery-speed hexproof very useful, at all? Maybe if you're doing it against a spell that your opponent hasn't drawn yet?

The ability of Psionic Invigorator is much more useful than this at sorcery speed (untap post-combat), but it would still be stronger at instant-speed.

This whole mechanic might have more design space at instant speed, but that does't mean it couldn't be sorcery speed, just that the abilities would need to reflect that (i feel like this one doesn't, the others are fine, tho).

Neat mechanic, either way you decide to take it!

I think it is. I mean, this drops with hexproof and can transfer it to another creature. It's also like a version of Crystalline Sliver for your psions. To be honest, this particular card might be undercosted and at too low of a rarity.

I've worded the reminder text as I have because I intended for moving the counters to be a special action, rather than an activated ability.

Why not Ψ counters?

I think tying the counter movement to a single specific timing (e. g. upkeep/untap step) would be preferable to avoid arguments about which counters have been moved yet.

Cards with Psionic: Master of Telekinesis, Psionic Invigorator