First off, I'm glad you like the new feature additions. We'd love to hear your feedback on Storage and FCM :)
Secondly, I'd love Carthage support as well, but there are a few things (built into Carthage) that prevent us from adding this:
1. Carthage requires iOS 8+ (dynamic frameworks only). Firebase supports 7+, and we build static "frameworks", so we'd lose that (which is still relevant in a lot of our developers).
2. Carthage is designed for open source (git based origins only), and while we'd love to get there, we currently ship everything as a pre-built framework.
Anyway, I continue to use (and enjoy using) Firebase for iOS and web.