I use it, it's fine as a "third place." Make sure you set the tier of the storage as the default Standard is fairly pricey. I use Standard-IA (Infrequently Accessed), which is still hot-readable but with a high retrieval charge—perfect for what should normally be write-only. A bucket policy ages out backups > 1 year. Encrypt your backups before you write them, of course.

There's nothing wrong with Backblaze et al. either. They may give much lower costs for bulk data. I only back up a few dozen GB so it's not worth opening another vendor relationship for me.

Somebody else suggested "hard drives at a relative's house." What's more likely, you lose S3 access—or your relative's house burns down / floods / is robbed / their kid yoinks a drive to use at school / any number of other disasters? Also, a backup should be as easy to do as possible—having to manually shuttle a drive back and forth from a remote location is probably not the best, unless you are extremely disciplined.

Suggest you research cloud backup solutions.

Myself I pay for Backblaze's Personal backup solution.

There also is Backblaze's B2 Cloud Storage (free 10GB tier) if you want something similar to S3.

S3 is subject to all the same bullshit. If you want to use S3, use S3... but you should still use that local drive as well.

buy two physical 12TB USB3.0 HDDs or similar and periodically rotate them through storage at a trusted family member's house off-site

The only answer for backups is Tarsnap.