i understand that you need to use mount,
i was sharing a real world command i use everyday with rclone, but with sync
rclone mount s3-awsbkpcog01:s3-awsbkpcog01 c:\mount\sqlbackup\20210427.144448 --vfs-cache-mode full --cache-dir=c:\path\to\cache\sqlbackup\20210427.144448 --log-level=DEBUG --log-file=C:\path\to\log\sqlbackup\20210427.144448 --config "C:\Program Files\rclone\.config\rclone\rclone.conf"
then point the sql server backup script to that folder.
as far as i know; one way or another, sql server going to have to backup to local system.
rclone mount is using --vfs-cache-mode so in effect, sql server is backing to to the local rclone cache and then rclone will move that to the cloud.
so i would choose to backup to local storage and then rclone move.
i do know that sql server can backup to azure, perhaps in the way you can get around the local system issue.
and take a read of this post
Sql Server backup to S3 (vfs-cache-mode)