Writeroom mode and linum mode are incompatible (leave numbers at far left, making them useless for (evil) jumping). Is there anything else I can do to get centered text with (relative) line numbering close to the text?
Centered-window-mode works with line numbers, though it doesn't provide the numbers directly next to the the text.
However, if you are using the latest emacs, it now has native display of line numbers. If you set
display-line-numbers-type 'relative or
display-line-numbers-type 'visual and use in combination with
centered-window-mode you'll get the numbers right next to the centered text, and with relative line numbers except for current line.