Just a week or so ago, someone posted a series fantastic bare bones granular synths. I can’t remember the users name, and I can’t find the thread. I took the liberty of attaching what I still have saved, I hope he comes forward so he can get the credit he deserves! There is also a granular synthesis in the example patches that is really great and has a ton of features. It’s well commented. I used it to teach myself granular synthesis by reverse engineering it.
I don’t know about nesting poly~ objects inside one another, and I haven’t yet tried it. I want to for the exact same reason as you though! Polyphonic granular synthesis would be a lot of fun. I know you can nest a pfft~ inside a poly~ and vice versa (there was a thread about it not so long ago) so I see no reason why you can’t put a poly~ in a poly~.