Greetings!

This is indeed a nice map you have here. A few things I noticed:

The grid, the scale on the border and the scale on the map legend are all not corresponding to each other in length. The scale on the border is also asynchronous, which shows at the bottom corners, where the yellow parts have to move around said corners to match up with the black parts. I'd suggest to redraw the border scale and make both the grid and the scale on the map legend the same length as that would greatly help measuring distances without having to look at the scale all the time - because in that case I know that one block of the grid is, say, 100 miles for example, since it's the same length as the scale.

As for the coastline, yeah, it's a bit thick and pixelated. but it's not too bad. I think you could do without the inner coastlines (the one on the land) and slightly (!) blur the rest. That should do.

And for the text you can either

select from alpha -> stroke selection with a soft, white* brush (not pencil) ~3px

or

duplicate the text layer(s), invert color and do a gaussian blur on the duplicate. Make sure that the original text layers are placed above the duplicated ones, though. This will give a slight glow to the text, which should make it a bit easier to read.

I think there are also specific glow filters if you have the additional filter package for GIMP installed, but I rarely use them.

(*or any color you like, but it should be a light-toned color in contrast to both the text color and the map color)