It’s still not sounding hopeful, but some web proxies need configuration in some browsers, while there are also transparent web proxies that force web traffic through them, so don’t need any browser configuration.
Do you happen to know which you have? This sounds like multiple customer sites, making it more difficult.
Possibly a backup program more aimed at businesses would support bypassing corporate firewalls better.
There are some third-party programs such as Proxifier (payware) that might be able to help get by firewall.
I just noticed your edit about WebDAV. That might make things a little bit more possible thanks to the .NET Framework HttpClient.DefaultProxy Property which takes environment variables such as HTTPS_PROXY.
Couldn’t connect to minio server when proxy configured in linux was the previous attempt at getting this up.
You could first try setting an environment variable to a no-auth-required web proxy before starting Duplicati.
Or if you can’t get it to work, you could take a system (if one exists) without a firewall in the way, and see if configuring it to a non-existent web proxy can break it. You then at least know it got noticed. If that fails then
If you can manage to get standalone rclone going using the similar scheme that it gets from Go, then use Rclone destination storage type for Duplicati. If need be (to keep Duplicati from also seeing proxy variable), rclone can be invoked through a wrapper script that passes along all parameters – plus the proxy variable.