Trim tends to be the first thing that rots... but if you are doing hardie trim then you probably mitigate that. The Azek may have more intricate profiles, etc if that is important to you.

Traditional houses have sills especially if they have that heavy crown detail above the window. Most don’t have exterior aprons so you could skip that. In most traditional houses the exterior sill, casing, and header casing all align (your illustration shows the sill and header casing projecting past the side casing which used to be seen only on interiors. )

What you do on the front of the house should be done on all the windows.

If you're keeping the more traditional style, then yes to the sills without an apron. If your look is more contemporary, then eliminate the rams head and the sill

" Traditional houses have sills especially if they have that heavy crown detail above the window "

" Nothing cheapens the look of the house more that reducing the details. Use the Rams Head Trim on ALL the windows. "

" What you do on the front of the house should be done on all the windows. "

Yes, yes and yes. I agree on all 3 of these statements.

"I had a client that eliminated the grills on a prairie style house on all sides except the front without letting me know. He regrets it now and the home suffers for it."

Sometimes you can't save homeowners from themselves, no matter how hard you try.

My neighbor was adding nicer trim to her house, added dentil above the front windows. It looked nice. Then it continued to the side and back windows, and it makes all the difference. Do the Ram’s Head on all the windows.

If the installed price is $100, you should do it.

The "apron" should be deleted to allow space for the siding to tuck under the historic sill (as intended). That would be more water resistant and look more historically authentic.

An apron is a trim piece that belongs on the inside under the stool.

The existing windows have a projecting sill that extends horizontally to the outside of the vertical casing trim so make the others the same.

Would you spend an extra $100 per window for Azek sills?

On the horizontal element that is most prone to weather damage and is essentially the 'foundation' of all the other window trim? Absolutely.