I believe Ceph [1][2] could be a good alternative. It can be self hosted and I believe some cloud providers also offer it. Here are some differences between S3 and Ceph [3].
Digitalocean Spaces uses Ceph as their backend and it is NOT stable (at least with s3fs), had to migrate away from it and could not do a full-stack on DO (aka "no AWS") and had to swap in Wasabi.
[1] - https://ceph.io/en/
[2] - https://docs.ceph.com/en/latest/radosgw/s3/
[3] - https://www.lightbitslabs.com/blog/ceph-storage/