I’d like to send an update, maybe someone would stumble upon the same issues.
After many tries to get this right, I’ve went for an approach to draw the hex tiles on top of terrain points, using the built-in SPS of the Dynamic Terrain.
Basically, the distance between terrain points are hex distances, and each point on the map would get it’s own hex tile. The performance is decent, and the memory thumbprint is not too big. When scrolling the frame time can get up to 10ms, but that can be mitigated by using some DT tricks (LOD, sub zones offsets).
The result is a bit like this:
And without the terrain mesh rendered:
Next, to sending some nice color/texture information to the SPS particles, and allow placing featues on top of the hexes.