To create free space if you are going to set maximum age of temporary files to 0 days, it might impact the operations of your system and its stability.
If you are unable to have available the required free space needed by Virtualmin’s backup module to create an archive before it is transferred to your S3 bucket then you will have to switch to a backup procedure that does not need to create large archives. You could consider https://rclone.org/ as an alternative.