A couple of duplicate shades. If you look through the neck area you'll notice you have several colours that look very similar but aren't the same.
Not sure if you know, but I'll state it nonetheless, FE-GBA has a colour limit of 15, with the sixteenth being the background colour for transparency. You mentioned FEXNA which won't have the colour limit but you'll want to hone down on it for style and aesthetic points, plus it helps with keeping it being a pain to recolour.
You also have a similar issue with the cloak in that a couple colours haven't been changed in certain parts, like you missed part of the recolouring.
The neck-seam comes more or less to personal taste but I prefer thinner lines rather than the thick and muscular lines like on the three you've spliced from.
Dieck, the middle character however, I personally feel has very dark features, and by that I mean he uses a lot of the fourth and fifth skin shade in his face. I feel you could probably get away with changing up the eye to make it use less of the fifth shade except for defining lines.
There's about a 2 or 3 pixel line on his beard where he's missing an outline.
The ear is filled with hair colour, losing it's definition and inner shaping.
The shoulder pad is also reverse lighted, where it's shadows are brighter than the top of the pad.
The shape of the hair worked for Astor(?)'s angle but doesn't for Dieck's, so I'd recommend adding in a tuft on the top left to balance it out or use a different hair-piece.
His inner eye is a pixel too high for the facial angle. Not too sure if it's an issue with Dieck as I didn't check but either way it's in need of a touch-up, just move it down a touch and fix.
When you fix all the colour issues, it gives you an extra few colours, so you can colour the pauldron a different colour. If you forego a white or light blue (which I use for eyes when I have a spare) you could also add a third shade to the cloak.
Now, just in case that all seems like a lot, have this and use it as a guide.
Disclaimer: I'm not the best, nor do I claim to be. I've just been at this a long, long, long time.
I also have a collection of my FE mugs in a zip over on Serenes, if you ever need a resource or reference I'm sure there'd be something in there. I'm in a process of updating them though, so for the latest just pop into my gallery on this forum.
P.S. Now that I look at it, I should have also moved the widow's peak over so that it sits in the middle of his forehead.
But that's just the nature of it, you'll always notice new things that you can touch up, art's never finished, as they say...