Both are possible in the future.
The difficulty with the pre-flight check is that it's very hard to know how much space the backup will consume on disk in its compressed form because it depends how compressed the source files are.
The challenge with streaming the backup directly to S3 is that the backup runs as the website user but the S3 credentials belong to the master organisation so we have to be very careful not to expose them.
Have you considered switching to the Enhance backup role? If you have more than a handful of websites it's a lot more efficient.