I am using MerakiMx84 Firewall provisioned with 2 WANs, 1st WAN is primary and 2nd WAN is on ready state as Backup.
With Help of Preference Internet routes, I setup some static Routes to S3 Bucket from WAN2 using any source and its working Flawless.
Right now Backup Circuit is kind of 90% Bandwidth utilization state and it will be like that for few weeks. and if we loose our Primary Circuit and Backup will be Primary, it will be issue as there will less bandwidth available for other traffic.
I want to setup some sort of policies, if Backup becomes Primary it will limit the Bandwidth for that AWS to 100 Mbps. I have the LAN source IP address which is uploading all data.
Through normal dashboard interaction you won't be able to do exactly that.
You can however play with priorities.
I would look into a periodic running script that polls the state of the uplinks or a way to catch an alert that triggers a script which in turn sets a traffic shaping rule that limits to 100Mbps for traffic matching your AWS destination subnets.