Dwarven chariots with ram teams ... now were talking!
The base damage + magic overlay system sounds like it will go a long long way to "filling in the gaps" between existing unit type appearances on the tech tree. This sounds
extremely promising.
One comment though, mostly just from a "flavor" basis. How about replacing (perhaps) Lightning with .... need a good term ... let's call it "kinetic" for now, because I don't want to call it "physical".

But the idea is to represent spells that do not directly blast the foe with magical energy, they blast the environment and the environmental splatter harms the foe.
I am talking about stuff like summoning a cloud of anvils 50' over the heads of a rank of advancing Macemen, or earth elementalists ripping boulders our of the ground and hurtling them at the enemy. Or your old-fashioned mentalist using telekenesis to fling convieneintly placed basketfulls of darts, or you air elementalist calling up dust devils and flying stones, etc.
Then you could have a "kinetic" (smallish damage/little resistance mitigation) and "energy" ((fireballs/lightning) bigish damage/large resistance mitigation)
That construct might give your "flavor designers" a bit more flexibility. (Or not.

I gotta find 2 cents to chip in. The approach sounds great.