Personal preference is fine and fat AP builds are definitely super fun, but ultimately just in terms of effectiveness Luden's/GLP into double pen is the objectively better build right now. There's a reason that if you go look at mage builds in high elo all the "traditional" mages like Orianna, Syndra, or Ziggs all buy Morello's second after Luden's. It's a faster and cheaper early damage spike against priority targets.

If you want to forego a LC upgrade for RoA first item your only option for matching the CDR is transcendence (easy, its the best mage rune in the tier) and then kindlegem or codex. Not only does it set you back an extra 600-700 gold just to match luden's, but all of those items push you into pretty specific item paths. there's only one item that builds from kindlegem for mages, and only four situational items for a codex. You can count the number of mages that want to build nashor's on one hand, Athene's is a support mage item more than anything, and outside of specific matchups and battlemages, and both Zhonya's and Banshee's are pretty poor second items. And that's assuming the Luden's mage doesn't take transcendence for some reason, most of the time you've spent more gold to sit 10% CDR behind them anyways.