Can anyone tell me if there is likely to be any problem taking artemesinin or milk thistle with tazemetostat in someone with mod/sev ckd + arrhythmia?

Don't use supplement: Avoid herbal supplements, and in fact all "dietary supplements" while taking medication. The best thing to do would be discuss why you are using milk thistle and artemesinin with your doctor, and see if there are any suggestions that they might have to help you deal with the issues you use them for. It's helpful to bring the bottles with you to the visit so your doctor can see the exact supplement