I think your proportions look pretty good actually. Just a few minor things really. The only thing that really sticks out to me in the head portraits is that the nostrils are too small. Nostrils are a bear to do with acrylic paint (at least in my opinion).
I don't have a ton of advice about acrylics because I am not nearly as proficient with paint as I am with pencil, but here are a few things that have worked for me so far.
Do you have different types/sizes of brushes? There are brushes that are square in shape and flat, brushes that have round tips, brushes with more pointed tips, fan brushes (I have one, but have no idea how to even use it...lol) and all these brushes come in different sizes. I have just played with the different brushes over the years and figured out which brushes work best for certain strokes, though this is still an ongoing process for me.
As far as mixing colors go, don't be afraid to use colors your wouldn't otherwise think to use. I will sometimes mix a little purple or blue with brown for dark areas. Someone once told me not to use black directly to creat shadows, but to mix blue an dark brown. This works for me most of the time. As with the brushes, figuring out color mixing combinations is also an ongoing process.
And then there's the old saying "Practice makes perfect."