You're very wise to have physical diversity for backups.
Buckets are pretty much public-open, aren't they? I use Google Cloud (GCS) for my websites, which are by definition open and their names are part of the URL.
For a few months I did a complicated fuse mount of a bucket and ran an encrypted filesystem in it, but it was so slow as to be unusable. And more costly, in that there's a lot of file thrash (read, write, read, write) and for GCS, at least, write and storage costs are nil, but *download* costs (modest) money.
How's AWS structured pricing? I thought they were competitively similar but I honestly have no idea.
I am using https://www.rsync.net for machine backups, using the rsync tool in linux. MacOS has it too. Windows is S.O.L., and honestly one of the reasons I ditched Win was its lack of rsync.