When I researched what to use for my backup I found rsync.net. They have some nice features nobody else seems to support, like they support ZFS send/receive https://www.rsync.net/products/zfsintro.html
But in the end the price made me go with borgbase.com
RFC 3339! ISO 8601 has way too many weird formats that are allowed like today would be 2023-W41-2. See for example here.