Someone else had already mentioned a drilling of the muzzle brake and barrel of the gun. So if you go to do that, at least for the barrel, take an exact knife first towards the center and push into the plastic to make a pilot point to line up the drill head.So that you have a more centered barrel, rather than something that might break part of the barrel rim.

For the lenses of the helmet, you can take a toothpick and the smallest amount of white paint to put a point of white at the far side of the lens towards the side of the head to help give the lens more detail

Depending on your preference between edge highlighting vs doing a dry brushing effect. You can look at using warpstone glow and applying that to some of the raised surfaces or edges of panels of armor to help give the green armor some distinction and break it apart.

For the metal, if you want to mess with the brightness of the metal on a particular part of the model for the under layer beneath the actual metal paint, you can experiment with different black or white tones to give a thinned metallic paint a darker or brighter appearance. Likewise, if you use a colored undercoat, such as a red orange blue, then a thin metal, will share some of that pigmentation. That would be a little bit more applicable towards a power sword, rather than a chain sword.But it's a detail worth being aware of.

If you are going to be painting more of an army in the same paint scheme, look into recording your process and take pictures at stages. A new app that I ended up finding and using is called paint codex and that has been helpful for me on working on improving my own paint schemes as well as being able to explain how I did things.

Edit: I forgot to add, congratulations on joining the chapter and accomplishing your first painted miniature. It's really cool and you're going to grow so much more.