Hugh,

I would double the 3/4 (treated?). You can also use non-treated even 2x12 etc. with protection (e.g. Tyvek). Sometimes this performs better than the treated. Also you might consider: 1) Rim Board -- expensive but can simplify things for 8" (13 1/4") and up no ripping and double bucking if you will. 2) You might also consider going back to how you've bucked all along and adding extra of 5 or 6" maze nails (twisted to grab concrete) and leaving your internal bracing in for several days to allow the concrete (and wood) to cure a bit. 3) You might consider V-buck as well. Regards.

Doubling the 3/4 ply ? Doesn't that bring you back to 2x12??

My favorite buck, for doors is 3/4" GT plywood attached to a 2x6 ripped at 22.5 degrees, and turned so the concrete gets behind the angle cut to hold it on. If that makes sense. That gives you 2.25" to attach trim and stuff.

On my own house we are trying three other methods too. No wood, 2x12 as finished buck for overhead doors, and spf 2x8 that do not go to the exterior.