I believe glass can greatly reduce the occurrence of this effect, though not entirely. I don’t think it is possible to eliminate this when using unions.
Might I ask why you want the glass to be this specific shape? You can typically get away with just one part, with the unwanted parts of the glass being hidden by the wall its fitted into, if that sounds like what you’re trying to do that is.
You can import everything in the picture into Blender, and create a custom window shape that fits what you need, then re-import it into studio. On import, it should be positioned in the exact place you intended it to go. If not, you can multiply the X and Z values by -1, which should put it in its intended position.
I consider unions bad practice, even if they are convenient sometimes. It definitely doesn’t hurt to know a little about modeling if you don’t already.
Unfortunately Roblox doesn’t allow you to edit Union faces individually (they aren’t a pure 3D object software, after all), so you have to get creative with some work arounds.
After only recently getting into modeling myself, I will say it doesn’t take long to learn how to do simple jobs like this. I would definitely recommend just learning movement controls, and some of the simple hotkeys like fill and extrude. It really can be a really useful tool that helps you get the precision and optimization that CSG can’t offer.