It seems like a self-defeating argument to claim that Venat's mistake was in not getting rid of Emet, Lahabrea, and Elidibus before they had the chance to destroy multiple worlds and end countless lives in the future. In a way, it assumes that the Convocation members had no agency or choice of their own because they were somehow 'destined' to turn to evil no matter what happened.

An important element of the Anamnesis Anyder recording was that Venat refused to speak ill of the Convocation despite their tyrannical conduct. The EE Vol. 3 provides further context to this, because we now know that there was a large schism in society over this decision and a significant opposition to the sacrifices that was deliberately ignored by the Convocation (p.11). She would have been quite justified in condemning them.

My impression is that Venat's believed in Emet and that he could overturn his own promised future. This did turn out to be the correct decision in the end.