I know how to do this.
Your army is as slow as the slowest unit. An army consisting of cavalry only, maybe a Roxolani hord, must be faster then a heavy Makedonian army with Hoplites and Pikemen.

When armies would march they would march side by side, even the largest armies consisting of cavalry, artillery, and infantry would move at a certain pace so they would arrive at the same time. And if you want me to do this for a mod (excluding DEI) add me and ill make it for you, or better yet i can teach you how to do it urself.

So cavalry armies get the same speed/range like heavy infantry? I don't think this makes sense.

And your mod affects only vanilla units, so units added by mods will be much slower.