Understandable concern, but AWS being targeted at enterprise gives me confidence they don't do any funny stuff like that. On and of course, they bill you monthly, so ideally you'd have a tell if something goes wrong.
I also store my files locally on a USB drive in alternative locations, but I conceptually trust S3, but maybe actions speak louder.
For cloud backup I use Arq backing up daily to AWS (no affiliation with Arq other than being a happy customer). You get client side encryption and the daily backup directly mitigates your concern, if there’s an AWS account issue you will know immediately and can fix it. For my storage amount and use it only costs about $2 a month.
I have about 80 GB or so of data being backed up. The daily backups upload only new files plus files that are changed. The largest monthly AWS bill I ever got was $6. The next month it went back to the usual ~$2 range.
While a scanned copy of something like a birth certificate or deed might not often work in place of a physical copy they're nice to have on hand. Plus they're a part of a family history.
i use spinning rust in x3 single usb external enclosures[1].
i mirror s3 data, both as an extra backup and to prevent s3 egress billing for random access.
s3 egress is for disaster recovery only.