The biggest single mistake had to be the flight to Varennes. The constitution of 1791 left him a good amount of power and at that point he still was favorably viewed by much of the population. He may have run into trouble with the Legislative Assembly (as he did OTL) but would have had much more of public opinion on his side.
But I think the Civil Constitution of the Clergy was too hard for him to take, when the Pope would not sanction it. Louis had finally agreed to it in the end but was deeply troubled. Pius VI’s stance put Louis in a difficult position that he finally could not tolerate. If Pius endorses the Civil Constitution I think that creates huge butterflies. Louis accepts his new situation and reigns as a constitutional monarch.
But I think the Civil Constitution of the Clergy was too hard for him to take, when the Pope would not sanction it. Louis had finally agreed to it in the end but was deeply troubled. Pius VI’s stance put Louis in a difficult position that he finally could not tolerate. If Pius endorses the Civil Constitution I think that creates huge butterflies. Louis accepts his new situation and reigns as a constitutional monarch.