Hi!
As far as weapon ranges, most weapons have much more range than those shields. Some (not all) of point-defense weapons don't, so in that case it's a choice you have to make - to rely on PD to take down an incoming missile cheaply but risk getting hit, or to play it self and take the hit on the shields - at the cost of more flux.
As far as flux, well, making it so that you can't fire everything at once is the point
You have to make a choice - which weapons to use in any given situation to use your flux capacity most effectively. Without a system like that, you wouldn't have nearly as many interesting choices - firing everything, all the time would be a very good strategy. That's fine for an action-oriented game, but Starfarer is a bit more tactical than that, and the flux mechanic creates deeper gameplay.