Your ideas should work, but it's less than optimal. Seems like it's not possible to modify metadata tags on S3 objects without copying the object, so there's no real hope of adding the mtime tag. Plus most of my data is in deep archive which would make that quite costly.
My goal was to move away from Synology-specific products so it's simpler to switch to a different brand of NAS. What I'll probably do is just reupload everything with rclone if I ever decide to move away from Synology.
Thanks for your help and input. At least I learned some things in the process. 