Maybe I don’t understand what you mean but I don’t think upload/download speed is irrelevant. If you use 1GB volumes and want to restore a 1MB file, Duplicati will have to read at least one 1GB volume file. So restores can definitely be slower with larger volumes.
Are you saying that because your backups are to a local disk? I suppose it would be less of an issue in that case.
As far as file count limits, you definitely won’t have issue with an ext4 volume. I’m thinking more of OneDrive back ends that are limited to 5000 files.