That's pretty much the one situation where they're competitive, so sounds very reasonable. Some of their competitors (Cloudflare, Backblaze) might compete, but the biggest cost issue with S3 by far is the egress so if not much goes out it might still be best for you to stay there.

Sounds like (unlike most people who use AWS) you've done your homework. It's great to see. I've used AWS a lot, and will again, because it's often convenient, but so often I see people doing it uncritically without modeling their costs even as it skyrockets with scale.