Yep, someone suggested MinIO in the comments https://news.ycombinator.com/item?id=26956330

Minio is not a caching layer, it's pretty much spinning up your own object storage. I have used it and SSDB, and SSDB is a better drop in replacement for redis. Minio is a drop in for S3 if you don't want to pay for S3 and not serving a large enough data.

Oh, in that case I still prefer using S3 because it scales and has built-in redundancy and reliability.

Yup, nothing will match the redundancy and reliability of S3. If you use Minio and have hard drive issue, you now need to restore. If you really want to cut down on S3 cost some mor then you might want to use Wasabi or Backblaze with Minio as the S3 layer in front. If you're profitable tho, the peace of mind of S3 is worth it than trying to save another $100.