With whatever mods you're currently running (don't remove them from your "Activated" JSGME list), go to your game's Silent Hunter Wolves of the Pacific/Data/Menu/cfg/Maps.cfg file. Make a copy of the original "Maps.cfg" file, and put it into its own group of folders. Create a new folder named MY MAP, then create another folder inside it named "Data", then another folder named "Menu" inside the Data folder, then "cfg" folder inside it. Put your copy of the "Maps.cfg" file into the "cfg" folder. Your group of folder will have this hierarchy: MY MAP/Data/Menu/cfg/Maps.cfg. This way you can paste the "MY MAP" group of folders into the JSGME "MODS" folder, and Activate it just like any other mod for testing.
Opening the "Maps.cfg" file with Windows Notepad, you'll find something like this:
The above is the stock game's file for controlling the Navigation and Attack Map behavior (your modded game "Maps.cfg" file will look a bit different....with different values for some of the parameters). I've highlighted the section you should try changing to suit your plan. Under the [Map0] section (the Navigation Map parameters), find the "ZoomLevels=" heading, then delete the first couple of zoom levels (say to the 50 level)......Use the "File" menu to "Save" your changes, then exit from the file. Open JSGME, and "Activate" your MY MAP mod listed in the left side panel (always have
ANY mod "deactivated" {removed} from the game when you make changes to it). Start the game and see what you get. You should find your closest zoom level is not as great as it previously was.
One thing you should try to avoid is changing the "ratio" of zoom between levels. I did this back in the early RFB days when I was helping out with SWDW. It made the harbor detail's shift slightly on the Nav Map, putting many of them on dry land!!