Honestly, Lux can always be a great pick even in high ELO when you're playing solo queue. Just make sure you're not shoving the lane too hard and messing up your AD's farm with your E. It's hard to poke well with her, but once you start to get the hang of her mana costs you can really become a large threat in lane. Lux can outdamage most supports in lane, and her q is great for setting up jhin w or jinx e.
Morgana's got more consistant utility with her black shield allowing you to avoid CC, but I think Lux can be much stronger when played well.
If your team needs peel or will need your shields, go Aery otherwise run comet. I would say go gathering storm if you don't think you can smash your lane, otherwise run scorch. Build the AP support item -> boots and upgraded supp item -> whatever AP items you like to build. If you're getting ahead early you can go for a dark seal and mejais later on, if you have a good frontline it's easy to rack up a lot of assists without dying. If you want to go more utility, Ardent Censer is fun and you can apply it to the whole team if they're grouped up with your w.
support lux is good in my eyes , real nice burst if you can land your qer combo
