Not sure why you wouldn't eliminate any window trim all together, and just tile around it, and then use one of the powder coated Schluter for any exposed edges.

I have no experience doing this but I will within the next few months; I am doing a similar thing. I wish you well and hope everything comes out great and you are able to share the results with me, I mean all of us.

" After waterproofing the opening, can I use silicone caulk to adhere? "

Absolutely. You may want to use a dab or two of an adhesive in the center and then a good quality silicone for the rest and perimeter silicone caulking.

What is your plan to waterproof the window (windows shed water hitting them from outside, not inside - the sashes, jambs, sill)?

@3onthetree, I'll be using cement board and Red Guard.

That is not going to integrate properly with your window. And it's the absolute worst possible choice. Most people don't even know what a mil gauge is, how to use it, or ever get the required coverage.

I read it as the window is within the shower surround, with fiberglass being stated because it is full size, like a double hung/awning/casement/slider, not a high transom outside the water spray. So shower curtain or glass in front of window to avoid water getting down into the wall via entering the window frame.

Also, it seems to me the adhesive for the PVC/Azek (Titebond, Azek, or similar) may not hold consistent on the RedGard or within water. Titebond specifically says not for continuous submersion or for use below the waterline, so in a shower niche there's just too much risk of water getting behind the "trim." But I haven't ever had trim adhered inside a shower surround, so I'm educated guessing.

What's the aversion to using the tile for the niche, or something like a prefab Kerdi niche?