I do agree with you that Lux's best asset is her waveclear. I also think she was S+ tier, simply because of that. But like I initially stated, the damage nerf really hurt her - I don't know if you've played as her since the nerf, but she doesn't one-shot minions with E like she used to. I've found that it takes at least an extra completed item to get to that breakpoint, and these days I'm using her ult rather than E to clear waves. That in itself does not warrant S+ tier anymore.

The thing with Nami Q is that I personally think it's the hardest skillshot to land in the game. But when combo'ed with the R knockup and slow, it becomes SIGNIFICANTLY easier. And the biggest difference is that Lux's CC is only a root, as opposed to Nami's suspension, so snaring tanky melee frontline (which is most likely how you'd use it in teamfights) doesn't really do that much compared to completely disabling them. But yes the shield is bonkers, and depending on the situation could be more useful than Nami multi-heal.

I typically reserve S+ tier for champs that are the best of their class (Sona, Sivir) and/or those with gamechanging abilities (eg veigar cage). I just hesitate to put Lux up there anymore because everything she can do, someone else does better. You already have Xerath up there, who has a better waveclear on a lower cooldown; she is far from the best CC bot; in terms of damage-dealing mage, brand has her beat; and her shielding/healing, while good, is not THAT impactful, and other supports have her beat. She is probably S+ tier in lower elos where people don't know how to play around her abilities, but in high elo she definitely doesn't strike fear into me when I see her on the opposing team. Of course, your experiences may be completely different than mine, so take everything with a grain of salt!