3rd spell level is the lowest that I'd take Polymorph - it's comparable in power to fireball for example. 1 CR/spell level is a decent scalable option, though I'd be curious to math out the damage effectiveness and see if my earlier change holds up.

I'd really avoid strength bonuses on Shifting. Check out my DPR of Homebrew Classes. Even small adjustments to strength would drastically impact DPR as it raises to hit and damage. Using the mechanics of Enlarge/Reduce would be fine though, as long as the mana cost is there.