I think the most realistic concern here is amazon scanning your backups. I totally get it. A lot of the reason for retaining total control over the storage medium is that what is being stored isn't strictly legal.
You should locally encrypt 100% of the information you store on someone else's system with strong symmetric encryption. In fact, I think Amazon's legal team would quietly prefer that you do this.
Dumping a bunch of bare files named things like:
THE.ODYSSEY.2026.2160p.WEB-DL.DDP5.1.DV.HDR.H.265-GROUP.mkv
into S3 buckets is probably a bad idea.