CZ121 is basically a 60/40 Cu/Zn brass. The high zinc content – as I understand it – gives it its free machining property, but also reduces ductility as evidenced by the discontinuous chip you get on turning or drilling.

I'd be rather surprised if you can do what you want to, though hot working might open up possibilities I've never explored myself.

I'd think it would be better to try to find some bar in a higher-copper brass like CZ108, which would behave much more like cartridge brass, respond to annealing and take cold forming successfully.

I suspect this'd be an expensive exercise, however – so if brass is absolutely necessary, maybe you'd test the possibilities with what you have first.

The thickest I have bent is 5/16" square section into a 2.5" circle. I did it hot but did also have a couple of failures before I got a good one.

I would think that one of your problems will be the work loosing heat as it is put into whatever you are using to bend it with/around.

+1 for David's suggestion.

You will need to melt it under a flux to stop the zinc burning off.