make a general project and then embed other projects, one for each country. As you suggested this will speed up future maintenance. But I think that all the layers of an embedded project cannot be edited, they are only readable, right? This could be a problem in some way
make single projects for each country. Maybe we can provide a template with a world raster or something like that, and then load the specific layers
make a general project and then provide Spatial bookmarks to speed up the zooming of the interested area. This could also be done for district projects within a country
We have a few options: