To mitigate the effect of S3 going down I use cross-region replication to replicate objects to another S3 region. If S3 went down in my primary region I could update the app config to write back to the backup region and update the CDN configuration to use the backup region as an origin.

I did that out of paranoia but it turns out this could happen to us. Does that sound like a sensible approach?

Fortunately all my company's stuff is in eu-west-1 which still seems to be fine.

I'm certainly not an expert here, but just to make you feel good (if nothing else), this is exactly what we did this morning (Melbourne time). Woke up to a bunch of flailing Lambda funcs on us-east-1. Luckily we're using Apex so cross deploying them to Singapore took all of 30 seconds. We were concerned about API Gateway since it was also sitting in us-east-1 but ended up not being an issue. Realized that redeploying S3 and Lambda across regions can be done in practically no time, but we would have been in trouble had we needed to replicated our APIs in another region. Going to start exploring spec'ing up our existing gateways in swagger to help with this.