A dip in liquid nitrogen and tap with a small hammer
Some things with epoxy resins are just not meant to be .
I would just cut them into tiny random pieces on a band saw . If the pieces are too small to be hand held then simply use some CA or dbl sided tape to attach to a piece if wood and saw away.
The other opportunity is to cleave the pieces over and over with a wood chisel and a hammer effectively chopping them up into small pieces.
Put them in a hydraulic press a few thousand pounds of load should be interesting some reference videos to give you some idea what you can do with a big a$$ press
And a whole bunch more ............
https://www.youtube.com/results?search_query=hydraulic+press+satisfying
At the end of the day some things with epoxy resins are just not meant to be easy.
calabrese55