First timer question, how do I know when to chop comfrey?

Long time lurker, first time poster.

I was able to get a few plants of comfrey from one of my coworkers a few months ago, after hearing so many great things about that plant on this sub. I put them in the ground and they have been growing since. I want to be able to chop this for a variety of things (great mulch, fertilizer juice, etc.) , but I’m not sure when to do it. I have a bunch of pictures of my seven or eight plants at various stages here.

Would someone be willing to give me some advice about when to chop them, and how far down, so I can chop them properly without harming them? Thank you!

Pardon the weeds, it’s been almost 100° most days lately, lol.