DNS problems with Wireguard VPN
This is a known problem in WireGuard / OSX:

matchDomains=[“”] doesn’t do what the documentation says.
Specifically, DNS servers are not used if allowed IPs isn’t 0.0.0.0/0.

For now, I am just manually setting DNS from a script:
sudo /usr/sbin/networksetup -setdnsservers Wi-Fi 1.1.1.1

Looks like Tunnelblick is doing scutil manipulations for openvpn (https://github.com/Tunnelblick/Tunnelblick/blob/master/tunnelblick/client.2.up.tunnelblick.sh) -- which is working on my mac