Hey, these are great! I do digital to and I think that what would be great for your drawing are some cleaner lines oorr no lines at all! I think for the cleaner lines you could either use the pen tool in either GIMP or ps or theres a program called paintoolSAI that's really good for drawing clean lines
It's not very expensive and they offer a free 30 day trial without having to pay anything. I think it would just finish the drawings off and make them look more professional
As would having no lines at all I think it would make them that bit more realistic. When I draw I do my sketch then fill in the base very carefully round the horse then take the sketch away clean up my base so it looks smooth and then for the subsequent colours I make a new layer then alt click between them (in photoshop) this makes it so you can't draw outside of that base layer so you get crisp lines
All this is just niggly advice your drawing are already fantastic!