MinIO is great. I use MinIO together with FolderSync (Android only) to automatically backup the photos from my phone to my local NAS. It runs a scheduled job every night and they're saved in the original HEIC format.
I've also used MinIO to mock an S3 service for integration tests, complete with auth and whatnot.
Tbh I wasn't aware of Syncthing. In this use case it would work just as well I suppose.
One of the advantages of MinIO would be the wide compatibility with other S3 storage services.
If my NAS had downtime while on holiday I could spin up a new bucket on S3/Backblaze/Wasabi and backup everything in a few minutes.
I've also used MinIO to mock an S3 service for integration tests, complete with auth and whatnot.