As usual, the problem is the uneven background, that severely restricts waht can be done. The usual technique to even the background:
- Duplicate the layer (it is best to start with a truly monochrome layer...)
- Blur the top layer until you can't see the text. In 2.8 you use Gaussian, but in 2.10 you can also use the median blur. I havn't got enough experience yet to tell which is best (media used in example below).
- Set the top layer to Grain Extract mode. This produces an image where the background is now a quite uniform gray.
- Create a layer using Layer>New from visible and use Curves/Levels to set the background to white, and adjust black for readability