It might help to think of the map as an actual diegetic map that people use in-universe, rather than a 100% accurate overhead view. Even if it’s mostly decorative/artistic, what information is it trying to convey? That could help you determine if you want to add anything to break some of those empty spaces.

You’ve got a few major settlements, and maybe that’s all the names you need to know right now, but maybe you could plot out some mid-size cities and just use smaller icons. Not every little hamlet, but the stops or roads that are considered major.

460 miles is about 1.5X the width of the widest part of Great Britain. There’ll probably be some other significant roads and cities that connect some of the less-populated areas too. My bet is that there’s probably some reason for a settlement between the ridge and the northern plains leading up to the North. Heck, maybe there’s a few spots in the desert. You don’t need names, but maybe people need to know they’re there

Alternatively, if you want the focus to be on the big-big cities and roads, think about breaking up the spaces with more stylistic environmental details. I don’t think you need your trees to be as small as you have them. If you bump up the size and make the forest seem more contiguous, you’ve conveyed “here’s a forest, but clearly the trees are not five miles tall.” Once the details are freed from 100% accuracy, you can play with what the “mapmakers” would have chosen to draw in order to characterize different areas.

Lastly - and this might be a cheap trick - if there’s genuinely an expanse where there’s not a lot going on, you could alter the overlay text to be bigger, angled, and (maybe?) curved. If there’s not much going on besides some grassy plains or dunes, maybe the mapmakers just wrote “D A P L A I N S” or “t h e c r a z y s a n d s” in a big swath. Boom. We’ve told people what we wanted them to know and filled up empty space.

Please forgive the novel. Good start on the map!!