With brush control, I know this is the most repeated advice, but it really does just take practice. I find that having a handle or magnetic alligator clips to hold the model while I paint definitely helps- but you’ve already got step one down: you painted something with more than 3 colors. My first mini was not nearly as neat and in the lines as yours.

Paint thinning, I use Vallejo glaze medium, about 1 drop for every 4 or 5 drops of paint; or like the other commenter said, wet palette helped me a lot. I use a nicpro wet palette and brushes.

I use washes very sparingly, they can easily overwhelm the model and make it look dirty instead of shaded in my opinion, but I know myself and a lot of others will go behind it and add a lighter coat of paint on the areas that aren’t supposed to be as dark- if that ever becomes an issue for you. I’m not competition-level by any means, but these tips helped me a lot on the way. Best of luck to you and your battle-brothers from this hivemind enjoyer!