Cannabis and tomatoes are said to be pretty similar in growth and nute requirements, etc. but I don't know if that also applies to root growth.I'm thinking the soil and roots have packed the outer edge and the inner hole.
This design may have worked well with tomatoes - do they have different root structure?
Your pot design should work though. I tried that style a couple of years ago before I really started experimenting with SIPs. Seemed to work OK but I used it on non-canna plants and they were no where as big as yours.
The rootopsy at the end will be interesting.