I think the Rylais is a good choice but i think you should consider adding archangels staff to the list of items because Sona can really pump up her AP by spamming all those auras.

I kinda have my questions about rylai's crystal scepter. This item works when you use an ability to hit your opponent. Because she has only two abilities to attack the enemy. Plus the fact she already has one slowing move when she uses power chord with song of celerity activated.

Yes, the scepter gives health, so it can be useful for longer survivals, and yes you may hit two targets at a time with hymn of valor. Although the hymn of valor hits two targets at once, it goes after the targets closest by Sona...

I like the idea, therefore +1
But I disagree on the usability of Clarity - if your team has many champions that need mana it's always good to be abnle to heal their mana. Also early game it can make a difference to get the kill or lose the kill because of going oom.

I also disagree about the starting item, Faerie Earring would be better to get philo stone earlier, because with the runes you have Sona definitely isn't too squishy. I know what I say, because I play her the squishy way early game without HP runes and with Meki Pendant. She'll be perfectly fine ;P

I was gonan say something about Shurelya's but I see you included it in the optional items section, so I have nothing left to say, except that good job. If you wana see how I Sona, check out my build ^.^ Quite different from yours.