You like creating new abilities, don't you?
Let's stick with improved grab and constrict here (with an acid rider on constrict).
I'm always in favor of sticking with common abilities, where possible, as many feat prereqs and other "add-ons" often use such things. Plus, it makes for simpler bookkeeping for the DM.
I'd be ok with just stepping up the dice size like usual, with the first step turning d4->d6. But what you have is fine, too.
Great damage potential, low AC, low hp. How about we boost natural armor to +4 or +5 and get it to CR 3? Maybe throw in unholy toughness or whatever it's called to improve the hp if needed?
Probably 12th level caster like a ghast.