Leashing is to do with the unit moving out of position to attack target. If Leash range is set to 0, the unit will never automatically move to engage a target, it will only move to engage when told to do so, or when something is already within range. This is how the Artemis, Destructor and other long range things are setup.

Default leash is if a unit is stationary, aggressive leash is attack move.

What you want to adjust is the Minimum and maximum attack range for each unit within the steering module, this is the ranges that the unit is "Allowed" to attack within, and will adjust itself relative to its target to maintain that range threshold. But this is quite complicated and doesn't seem to work the way you want it to, I spent a great deal of time trying to perfect how artillery units handled their attack range for the 2.2 patch which only slightly improved it. In order for the Artemis to not automatically close in and instead fire from max range, you need to set the minimum attack range (within the steering module) to whatever the attack range of the weapon is, (I think this is 1400.) but when you do that, the Artemis will automatically back away from enemy units, so it practically becomes impossible to kill with anything other than air units. This was the case before with the Chronus and it was quite frustrating so we lowered it.

You can play around with it, but I don't think you're going to get it much better than it is now. (Unless I was doing something seriously wrong when I tried.) We're aware it's currently not a very robust system and having artillery units close instead of properly utilising max range is frustrating and makes long range units less meaningful. We have some ideas on how we can fix or mitigate this, but it will require some code changes to the game. This will get visited at some point.

I think if you want to change speed go to Assets>Formations>FlockerTemplates and edit Max Velocity.

Thanks