Ok so I have made some developments:
20240916-0307-49.9922231.mp4
In the above are some nice UI improvements:
Instead of the arrow, a rally point cursor appears when a unit's destination is just to a particular location.
When all a structure's firepower (building arrows AND degarrisoned soldiers) are to be sent to a particular enemy unit, the familiar sword icon appears. This is the behavior if no hotkeys are involved.
If you press ctrl ("autorally" hotkey, for example used to garrison trained soldiers back into the selected building), the rally point alone is set to that enemy and building arrows remain untargeted
if you press "force attack" (unfortunately this is also ctrl by default), you are in command of the building arrows and only the building arrows.
shown by the red crosshairs.
Also, the arrows now fire in sort of volleys or bursts, rather than a constant stream of arrows. This means they can be dodged to a degree, which is fun.
Next steps:
change force attack default hotkey (or find another appropriate hotkey).
add some audio feedback for when the arrows of a building are being targeted.
rebalance building damage according to non-random building AI.
rework building technologies and/or change building damage with each phase.