The most efficient tool I know of for cutting nice square window and door wells in panels is a chain saw fitted out with a “lumber maker” a readily available simple accessory which attaches to the bar, essentially a swivel and a piece of channel which is intended to ride a 2-by, marketed to be applied to a sawlog, but in this instance temp screwed to the wall / panel.

I’ve never done this as a retrofit, and like Pete suggests your challenge is avoiding fasteners, you have the additional layers of drywall (I’d remove to the line beforehand)and siding, both of which will make it harder to determine where the splines and their screws are hiding. Remove the old bucks (sawsall work-cut the nails) and the seams will be there to see.

Luck with it & be your best