WireGuard policy-based routing

This question is about configuring a WireGuard relay that routes all its peer's traffic to another WireGuard server, but the relay itself does not use that WireGuard server as the default gateway.

I am doing some self-hosting stuff. Currently my network contains three nodes, a gateway, an ownCloud server and my mobile phone. These nodes connected in mesh. The gateway is hosted on a VPS, and the mobile will use it to access the Internet.

Everything is working. But the problem comes when I add a Windows node to this network because the kill-switch function of the WireGuard Windows client requires the configuration to have exactly one peer and the allowed IP is 0.0.0.0/0. For security reason, I don't want the VPS sit between the Windows node and ownCloud server, so the route should be:

Windows node -> ownCloud server -> Gateway -> Internet

Moreover, the ownCloud server will run hourly restic backup. I don't want this traffic to route to the gateway because of speed.

I have been trying policy-based routing for around a few hours, but still cannot make it work. Could anyone help? Thanks. Below are the running configurations.

Gateway

wg0

[Interface]
Address = 10.0.0.1/32
ListenPort = 51820

[Peer]
PublicKey = (ownCloud server's public key)
AllowedIPs = 10.0.0.2/32

[Peer]
PublicKey = (mobile's public key)
AllowedIPs = 10.0.0.3/32

nftables

    chain postrouting {
        type nat hook postrouting priority srcnat;
        ip saddr 10.0.0.0/24 oif eth0 masquerade
    }
    chain forward {
        type filter hook forward priority filter; policy drop;
        ct state established,related accept
        ip saddr 10.0.0.0/24 accept
    }

ownCloud server

[Interface]
Address = 10.0.0.2/32
ListenPort = 51820

[Peer]
PublicKey = (gateway's public key)
Endpoint = $gateway_ip_address:51820
AllowedIPs = 10.0.0.1/32
PersistentKeepalive = 25

[Peer]
PublicKey = (mobile's public key)
AllowedIPs = 10.0.0.3/32
PersistentKeepalive = 25

Mobile

[Interface]
Address = 10.0.0.3/32
ListenPort = 51820
DNS = 10.0.0.2

[Peer]
PublicKey = (gateway's public key)
Endpoint = $gateway_ip_address:51820
AllowedIPs = 0.0.0.0/0
PersistentKeepalive = 25

[Peer]
PublicKey = (ownCloud server's public key)
Endpoint = (ownCloud server's domain name):51820
AllowedIPs = 10.0.0.2/32
PersistentKeepalive = 25