This is looking pretty sharp. I like the painterly style and dig the black and white thing you've done. Rivers get a bit odd as mentioned. Never too late to ret-con those, now that you have the benefit of the River Police's gentle tutelege. Are the geographical borders defined enough that they can be drawn on? Might be worth doing. Also, city symbols, points of interest, etc. would add to the map, IMO.
Since this is supposed to reflect, in-game, the style of a "collector's map" I think you'll want to think about what such a person would value highly. Looking at some of the maps of this type from our own world you can see that they tended to be highly decorative with fancy borders, heraldry, mystical things out where nobody has explored yet, etc. BUT they might do things differently in your world. Maybe maps are more like art and the cartographers are in their impressionistic phase; so they use highly accurate costal representations combined with impressionistic rendering methods. Might be a good way to think about it...and a good reason it's being handed off to your PCs at this point. Heck you might even be able to make it into a MacGuffin. Maybe somebody wants to steal it because it's so valuable - a one-of-a-kind Carto Famoso original! :)