Idea for a Lux change: Give her increased attack range if she's attacking a marked enemy Champion
I know this doesn't address her waveclear issues, but this is something that I think could reinforce her "safety" gameplay identity.
This buff would be akin to Caitlyn's bonus range against targets affected by her traps or net, though obviously not to such an extreme. Maybe something like 100-150 extra attack range for Lux if she's attacking a Champion affected by Illumination (Caitlyn, by comparison, doubles her 650 AA range). The buff's purpose would be to allow Lux to safely proc her passive on enemy Champions once they've gotten their ultimates, as she normally cannot attempt it without putting herself in suicidal positions. This leaves her passive feeling... unimpactful past early-game.
If other adjustments would be necessary, I'd say it'd be worth the extra safety.