In order to reduce thermal bridging but still have a solid (and inexpensive wood vs custom foam buck) surface to install the window and exterior trim to, I am proposing only having the exterior foam cut out for the buck. The buck would then we secured with horizontal screws through the interior foam and with typical vertical screws through the exterior foam. Will this work? Picture attached.

Afterwards, I plan to install foam on the interior side of the buck in the window well to reduce “buck  related” thermal bridging and then cover with wood trim. I have already checked that the 5/8″ trim against the sash will not interfere with the window handle.