You should keep in mind that S3 really is an object storage. It's named as such, advertised as such, priced as such and the limitations you're hitting are because it's built as such.
It does work if you want to host a static site, and it's nice that they offer a bunch of extra niceties to help make those work... but expecting things like user-agent-specific redirects is a bit much for what's essentially a filesystem.
I mean they let you put S3 behind Cloudfront with a TLS cert. Having read through all the documentation, hundreds of blog posts, etc., I've seen absolutely nothing that indicates that S3 + Cloudfront isn't meant for serious web hosting.
Amen! Have been baffled by the urge to move stuff to S3 for app hosting. I understand the convenience (sort of) and the scalability aspects (mostly) but you seem to lose loads of functionality.
It does work if you want to host a static site, and it's nice that they offer a bunch of extra niceties to help make those work... but expecting things like user-agent-specific redirects is a bit much for what's essentially a filesystem.