I agree. Just need some calm time to think about it properly!
In the mean time you could use the tardigrade/Storj backend which has erasure coding built into the protocol... The price per GB is good too.
It would be nice to be able to do that.
I guess there could be an rclone option --vfs-sync-on-close let's say which means make sure all files are uploaded before unmounting. I'm not sure how well that would work though in practice as things get upset (the macOS kernel in particular) if you take too long doing FUSE operations. Then again, we could shut down the mount and carry on uploading stuff in the background. So maybe that is two options --vfs-sync-on-close hard or --vfs-sync-on-close background, maybe...
The other way would be to make an rclone rc command which showed the number of pending uploads. In fact we added one recently vfs/stats in the latest beta
You could use this to query "uploadsInProgress" and "uploadsQueued" and only unmount when both are 0.