How do I seperate this HUGE zz plant?

I just purchased this monster of a zz plant (only $20- bargain!!). It’s gorgeous and very heavy. It needs repotting but ideally I’d like to leave most of it in the pot and just seperate it/thin it out a bit and replant the stuff I pull out.

I don’t want to pull the whole thing out (physically I’d struggle anyway) only to replant most back in. It’s been suggested to just use a serated knife (it will need to be huge!) and cut some out, taking the root ball with it?

Thoughts? Advice?