The gun is pretty big already. I don't think I would make it any bigger by adding a red dot sight of any kind.
I think you will really like it. The 21 was the first Glock I ever bought and one of my favorites.
Accurate and soft shooting.
I have the original night sights on it that were installed around 1998 or so.
I used that gun for my CCW qualification.
You did good.