I’m sorry if I wasn’t clear before. I only meant donations after the person’s heart had stopped beating for everyone
Cessation of breathing is the criterion (R’ Dovid Feinstein in his father’s name).
Since, as other posters have pointed out, most organs are only useful if harvested from a live person, so your statement, “that any Posek who refuses to let people do this is Machshil es Harabbim and Gorem Sakanas Nefashos for many Jews” is baseless.