Wouldn't it be better to use blocks (219 in ASCII) as the main paint unit and use some other half-block and "hashed" characters (ASCII 220, 223, 176-178) to implement dithering?
Everything above 127 would be extended ASCII instead of strict ASCII, if that matters.
By leveraging characters, you can actually do a form of dithring! See this great interactive blog post: https://alexharri.com/blog/ascii-rendering