Remove all the casing except the top. Install a sill with returned ends long enough and wide enough for the side casings to die into it. Cut a matching apron with returned ends as long as the side casings are wide and install it below the sill. Old school.

You have nice trim that I assume matches the rest of the house. I would be sad to lose it. But that is probably the correct way to fix the problem. Would it be possible to find a tile edging that would fit the small space?

Or you could add a small shelf under the window with a few small plants and cover the gap. This solution really shows my laziness :)

Just get a pencil trim and use that to cap the whole thing. I wouldn’t lose that original molding.

Maybe remove the trim and lower it Or get linger casings on the sides.

I would have done the tile lower. We cant see the whole job. Can the top row be removed?

Should have used Schluter

That's what I was just going to suggest! Corner blocks belong on the top of the frame, and should not be on the bottom. A sill is ALWAYS the best option for the bottom of a window, inside or out. Looks beautiful now!

Nice work!

Great solution. Looks good!