I have got one method which works but is quite limited on how the map needs to be made in unreal, so keep this as an example in mind. Funny part is it is very similar to your method when i look at the code. My map is made from a world composition ([tutorial][1]) which allows me to divide my final map into smaller submaps. But this technique also just applies to normal terrains made from grids. I total i use a grid of 3x2 for this drawn map (3 cols, 2 rows). Each terrain block is 127x127 Quads. The map drawing is 1920x1080p
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First you need to divide your source image into smaller blocks of (640x640 px). I personally use this [tool][3] and then choose “split both” with 640px for the horizontal and vertical resolution. Import all the split images as textures and keep their original name so it becomes easier to distinguish between them.
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Now you need to create a master material with this layout
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After you made the master material, create the n instances of the master material with the belonging tile name. So for example (mt_map_1_1, … mt_map2_3). This is so you can more easily adjust parameters to make the edged align in your final map.
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Assign each terrain block with the correct material instance (so top-left to mt_map_1_1, etc). I would also recommend switching to a Top down perspective so you can see uf the map is displaying correctly.
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You should probably see that the image is somewhat scaled correctly on the 3x2 terrain but the edges are not overlapping correctly. To fix this, open the Material instance of the terrain block and adjust the the value of the Diviser to fix this. You should avoid changing the Scale parameter unless you see this help with fixing the edges.
If everything went correctly you should see that you now have a 3x2 map that looks like this. (i personally had to change the Diviser values of mt_map_1_3 and mt_map_1_3 to 15.555)
But just to be clear, this is not a 100% correct solution as you can see some of the lines are a bit ■■■■■ and the scaling is not realy correct, but just as reference material so you can sculpt your world outline this should suffice.
Please contact me on if you can’t make it work, i would love to help!