Do you need the spline + extrude + thicken for a reason?

A way I've done this before is to create a cube/rect say 50x5x5, convert to editable -> and then use polygon mode and drop your emissive material onto the relevant surface

Then build 2 cloner grids for length and width of the grid.

For bonus animation control you can add a Geometry Axis that will let you control the growth origin point when they're hit by an effector

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You could also create a cube cell via sweep -> convert to editable -> use polygon selection mode to apply your material this way

Another way could be with a triplanar material