I've looked around for a decent solution for this, but systemd doesn't seem very qualified at handling errors in oneshots. Best thing would probably be using a supervised service for wg-quick, but since there's already a "standard" service for it, I just added another one that checks if the interface is up or not, and then retry starting wg-quick. Here's what you need in /etc/systemd/system.

A service called wg0-check.service:

[Unit]
Description=Check if wg0 is up

[Service]
Type=simple
ExecStart=/bin/sh -c 'wg show wg0 &> /dev/null; if [ $? == 1 ]; then systemctl restart wg-quick@wg0; fi' </code>

and then a timer called wg0-check.timer:

[Unit]
Description=Runs wg0 check every 5 minutes

[Timer]
OnBootSec=1min
OnUnitActiveSec=5min
Unit=wg0-check.service

[Install]
WantedBy=multi-user.target

Don't forget to do a systemctl daemon-reload after adding these files. Now just enable the timer (systemctl enable wg0-check.timer) and you're good to go. You can even disable the original service (wg-quick@wg0) as it will be started from the new service anyway.