Create a Router and route all outbound traffic thru it

[lavaquark]

I'm trying to set up a router in Proxmox, ultimately this router should be connected to a VPN, and all traffic from any container inside Proxmox to the internet should route through there.
To simplify it, there is the host on vmbr0, some containers on vmbr1, and the router (10.10.100.250) on vmbr10. Externally I have one IP for the server(lets say 192.168.1.2) and one gateway (192.168.1.1) assigned. If I set the vmbr0 CIDR to 192.168.1.2/24 and gateway to 192.168.1.1 everything works fine (but all traffic goes through the host and directly out), i.e. container1 -> vmbr1 -> host -> vmbr0 -> internet
I can now remove the vmbr0 CIDR and gateway and assign it to vmbr10.
But how would proxmox know to route traffic like that now?
container1 -> vmbr1 -> host (->vmbr0?) -> router -> vmbr10 -> internet
If I try to assign vmbr0 gateway to the router IP 10.10.100.250 (so the vmbr0 should connect to the router), I get the error: "Default gateway already exists on interface 'vmbr10'"

 

[UdoB]

"Default gateway already exists on interface 'vmbr10'

I won't reverse engineer your wall of text, but: while you can have a lot of routes to different networks you can only have one Default gateway.