This can be addressed several ways actually.
1- From an UI PoV the Icon could change in Color/ Design or have a scope icon over-lap it to show it's now empowered. It's not hard by any measure to show something like this, even less so with a role that ask the player to look at the hot bar for procs.
2- They could just do what they're already doing and let people figure it out with practice. Like Slide Casting, it's a thing that allows casters to play more optimally. Does the game have a line in the cast bar telling them "Ok, you can move now!"? No, you just gotta figure out when it's safe to move without interrupting your cast (not to say they wont do something like that down the line. There's already illegal add ones that do that for you). Same for ... Dancer. As a Dnc I don't know exactly how close I have to be to hit something with my dances or my AoE procs, but it is something you get a feel for. Sometimes you don't want to chance it so you go full melee for a bit in your burst cause you can ... some times you risk it and you're meet with a big sad as your bloodshower or dance does nothing cause you were a lil too far away.
Also, having skills not go off if a target is too close is a hard no, it gimps pure solo Ranged PvE play. Mnk doesn't suddenly stop being able to do their rotation because they can't get to the back of a mob that's aggro on them.
As for Mechs in fights, asking a DPS to consider their distance from an enemy in their play doesn't doom them to die to mechs any more than asking them to consider their position around the near area of the boss. Sure, some people might get lost in the sauce and tunnel on it ... happens with melees all the time. So what? No boss fight is impossible for melees or Blm and no boss fight forces ranged jobs to be in melee range for whole fights.
(Also, nah, Chain Lightning doesn't require you to be close to the boss. PLAYERS made a STRAT that's popular and fairly safe that puts everyone close to the boss ... the Mech it's self? It works on a system of "Chain goes to who ever is closest". As a Ranged you can be 20ft away from someone and it will bounce to them so long as they are the closest target. This is why Triangle Strat is more fun for me as a Dnc, I can have 2 people sit still and do the mech for them instead of risking some non-sense cause someone moved too close. Even with circle strat I move OUT and to the side to pass the chain to someone so that there's no chance of them hitting me with their small AoE)