Agreed, it's not rootbound, yet. It is certainly time for a new home though.
I'm wondering about your light too... I saw your comment that you're using an LED panel with red and blue. Sounds like more of a flowering light but you should be getting better growth than an inch a week.
I'd get a T5 and call it a day... money being no object of course...
Head scratcher so far.... keep us updated.