How to finish around the inside of new windows after replacing old ones
Get some 1x4.  I'd go with primed fingerboard pine.  Cut to size and install against the metal frame of the new window.  Caulk, Paint.

I've installed windows and then completely hidden the window frame with interior trim such that you only see the trim I installed and the glass.  The frame that holds the glass is hidden with the trim that butts up against it.