Why bothering with veticles if you can simply do this with normal pixels. You can always stretch that texture to fit the desired size if needed.
Why not change minimap texture as soon as you do any change to the map? Since you are usually changing one tile at a time this means that you only need to change one pixel or smal portion of your map if it is generated from map-specific mini sprites. I mean there is absolutely no need to redraw the whole map but only change the relevant part of it.
As for your suggested approach that is more suitable to be used inside a game where map changes are more rapid (unit movments etc.) but in map editor there is no use of that.
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