> The ":s" verb for easily viewing the size of directories / files

It's the keybind "S" with nnn (or "A" for apparent usage). You can also use the same option ("-S") to open nnn in du mode.

> the overview of multiple directories at once

I explained the nnn design strategy in my other response

> the gitignore handling

This can be added but it changes the current generic way of handling directory entries

> There might be more, but I would have to spend more time with it

That would actually be a constructive data to improve nnn as well.

Thanks for the notes!