Thank you! This is so cool! :D I made a GitHub issue for your request: https://github.com/fabioarnold/MiniPixel/issues/32
For now you can try holding shift to draw lines and alternate between pen und bucket by pressing N and B if that is any help to speed things up... I understand having different brushes will be really helpful when sketching or working on large pieces.
Looking forward to seeing your work! :)