Played around with different Blend Mode settings and a tried and true texture, also throwing in some RGB Matrix Process in the bottom two (Sepia and Gray respectively) with some minor colour adjustment. Dialled down the transparency of the backdrop brown "paper" for a more darker tone to "tame" the Sepia in particular. For the Grey iteration different colours would probably be required to make the hill look a little less wonky.

As with the Grey, I think better colour selection can make it work even without the texture (below)

...which is kind of neat because keeping multiple sheets for different Blend Mode settings (with or without RGB Matrix Process) basially means I get to switch between very distinctive looks for the map with a very minimum of effort (i.e. hiding "filter" sheets not in use).

So...yeah...gradience was less of an issue with this approach meaning fewer polygons overall - and consequnetly less time for colourizing. So I had no choice but to dabble with the time-sink that is the practice of highlighting. Obviously.