I've no experience with root-pruning but know that bonsai artists use the technique. I think you should google it because I'm not sure what precautions to take.
If you want to go a simpler route, I'd buy a bottle of Cannazyme and use that. It frees up the nutrients bound up in old, dead root matter within the soil and buys a lot of time. It also effectively gives a bit of space. I use it on my Christmas cactus and they like it too.
Just a bit of caution, after applying Cannazyme, CUT BACK on your nutrients because there will be extra in the soil!!!
I still think that your moms need to be regenerated. My friend has a commercial-sized grow and also needs high numbers of clones on a tight schedule, but he keeps a bunch of mothers that are no more than 3' high and replaces them when they look scraggly. Having extra moms allows for a failed run of clones, and more time for the mothers to recover or get big enoug to use.